feat: expand completion snippets and navigate placeholders

docs/configuration.md

@@ -50,12 +50,12 @@ Registered command names may contain dot-separated segments, each starting with
 Key notation follows [Herdr's readable style](https://herdr.dev/docs/configuration/#keybindings). Literal non-control Unicode characters are case-sensitive (`n`, `J`, `+`); named keys and modifiers are case-insensitive:
 
 - Named keys: `esc`, `enter`, `backspace`, `delete`, `left`, `right`, `up`, `down`, `home`, `end`, `tab`, `space`.
-- Modifiers: `ctrl+a` through `ctrl+z`, and `shift+a` through `shift+z`. `shift+j` and `J` address the same key.
+- Modifiers: `ctrl+a` through `ctrl+z`, `shift+a` through `shift+z`, and `shift+tab`. `shift+j` and `J` address the same key.
 - Named punctuation: `minus`, `comma`, `ampersand`, `plus`, `backtick`. Each addresses the same binding as its literal character.
 
-`ctrl+i` aliases `tab`, and `ctrl+m` aliases `enter`, including inside leader sequences. Ctrl+h and Ctrl+j are distinct control keys, not Backspace/Enter aliases. Only a single modifier on an ASCII letter is supported. Arbitrary non-leader key sequences, Alt/Cmd/Super, combined modifiers, modified special keys, and Vim-style angle-bracket notation are rejected rather than silently treated as another key.
+`ctrl+i` aliases `tab`, and `ctrl+m` aliases `enter`, including inside leader sequences. Ctrl+h and Ctrl+j are distinct control keys, not Backspace/Enter aliases. A single modifier on an ASCII letter is supported, plus Shift+Tab. Arbitrary non-leader key sequences, Alt/Cmd/Super, combined modifiers, other modified special keys, and Vim-style angle-bracket notation are rejected rather than silently treated as another key.
 
-Normal mode binds `ctrl+o` to `config.core.jump_back` and `ctrl+i`/`tab` to `config.core.jump_forward`. Insert-mode Tab remains literal text. Override or disable these keys with the ordinary per-mode binding interface; both jump actions also appear in the command palette.
+Normal mode binds `ctrl+o` to `config.core.jump_back` and `ctrl+i`/`tab` to `config.core.jump_forward`. Insert-mode Tab inserts literal text outside snippets; inside one it moves to the next argument. Override or disable these keys with the ordinary per-mode binding interface; jump actions also appear in the command palette.
 
 The built-in `gt`/`gb` sequences move to the first/last line in Normal and Visual; `gf` follows Markdown file links in Normal only. This is not a configurable general sequence prefix: bind `config.core.document_start`, `config.core.document_end`, or `config.core.follow_link` to a single key or leader sequence to add another route. A configured or disabled `g` overrides the built-in prefix in that binding mode; choosing `g` as the leader overrides it in both modes.
 
@@ -69,7 +69,7 @@ The built-in `gt`/`gb` sequences move to the first/last line in Normal and Visua
 | Normal | `follow_link` |
 | Normal | `jump_back`, `jump_forward` |
 | Normal | `lsp.definition`, `lsp.diagnostics`, `lsp.hover` |
-| Insert | `lsp.completion` |
+| Insert | `lsp.completion`, `snippet.next`, `snippet.previous` |
 | Normal | `search`, `paste_before`, `paste_after` |
 | Normal | `paste_system_before`, `paste_system_after` |
 | Normal, Visual | `select`, `select_linewise`, `search_next`, `search_previous` |
@@ -206,11 +206,13 @@ Diagnostics appear as severity-colored gutter numbers and text underlines; the p
 
 Hover accepts plaintext/Markdown MarkupContent and string/code MarkedString forms, including arrays. The popup grows up to 100 columns and 24 rows, clamped to the available document workspace and content height. Multiline content and Markdown markers remain visible, with the same Markdown syntax colors as document buffers; named fenced code blocks use their language's bundled highlighter. Highlighting is cached across scrolling and redraws. Horizontal scrolling exposes long lines without splitting graphemes. Missing/unsupported/invalid hover results report a message without changing text, selections, or undo history. Content is bounded to 4 MiB and terminal controls are sanitized.
 
-Completion is manual and uses only the primary insertion caret. Ctrl+n requests it; the first candidate replaces text immediately. While the list is open Ctrl+n/Ctrl+j selects next and Ctrl+k previous, including an Original text row that restores the original document and carets. Other keys close it and act normally, with no accept key: Enter inserts a newline, Tab inserts a tab, and Escape returns to Normal keeping the displayed candidate. Secondary carets are translated, not completed independently. Cycling belongs to the pending Insert undo group; repeat records only the final chosen edits. See [editing behavior](editing.md#manual-lsp-completion).
+Completion is manual and uses only the primary insertion caret. Ctrl+n requests it; the first candidate replaces text immediately. While the list is open Ctrl+n/Ctrl+j selects next and Ctrl+k previous, including an Original text row that restores the original document and carets. Other keys close the list with no accept key. Snippets expand the whole template and highlight the first argument: typing replaces it and its mirrors, Tab/Shift+Tab moves next/previous, and the final `$0` ends navigation. Outside a snippet Tab inserts a tab; Escape returns to Normal keeping the displayed text. Secondary carets are translated, not completed independently. Cycling and argument edits belong to the pending Insert undo group; repeat records resolved text and caret placement. Rebind the Insert-only `config.core.snippet.next` / `config.core.snippet.previous` actions normally. See [editing behavior](editing.md#manual-lsp-completion).
 
 The first assigned server must advertise a completion provider. Vamp handles item arrays and CompletionList results, plain text, TextEdit, InsertReplaceEdit (using its replacement range), supported edit-range/format/mode defaults, and atomic additionalTextEdits in the same document. Insert-only fallback text replaces the identifier prefix before the caret; `filterText` and stable `sortText`/label ordering are honored. All edit coordinates are validated at UTF-16 and grapheme boundaries against the immutable request document; malformed or overlapping edits report a completion error without retiring a healthy server. Aggregate completion text is bounded to 4 MiB. Pending replies cannot reopen completion after intervening input, source changes, superseding requests, or modal overlays.
 
-There are no automatic triggers, snippets, commit characters, server commands, indentation adjustment, or completion-item resolve requests. Items requiring snippet expansion, commands, or indentation adjustment are omitted rather than partly applied. Server-specific availability depends on the installed server's advertised capabilities; configuring a preset does not guarantee completion support.
+Supported [LSP snippet syntax](https://microsoft.github.io/language-server-protocol/specifications/lsp/3.17/specification/#snippet_syntax) includes numbered tabstops, nested placeholders/defaults, mirrors, escapes, choices (first option), variable defaults, and `TM_SELECTED_TEXT`, `TM_CURRENT_LINE`, `TM_CURRENT_WORD`, `TM_LINE_INDEX`, `TM_LINE_NUMBER`, `TM_FILEPATH`, `TM_FILENAME`, `TM_FILENAME_BASE`, and `TM_DIRECTORY`. Insert has no selected text, so `TM_SELECTED_TEXT` is empty unless a default is supplied. Unsupported regex transforms, unknown variables without defaults, repeated/nonempty final `$0`, or final stops nested inside arguments omit that candidate. Malformed templates report a completion-local error. Each template and expansion is bounded to 256 KiB, with 4096 parsed nodes, 1024 placeholder occurrences, 32 nesting levels, and a bounded rendering traversal.
+
+There are no automatic triggers, commit characters, server commands, indentation adjustment, or completion-item resolve requests. Items requiring commands or indentation adjustment are omitted rather than partly applied. Server-specific availability depends on the installed server's advertised capabilities; configuring a preset does not guarantee completion support.
 
 Semantic highlighting is **enabled automatically** when the first server advertises [LSP semantic tokens](https://microsoft.github.io/language-server-protocol/specifications/lsp/3.17/specification/#textDocument_semanticTokens). No additional configuration or binding is needed. Full-document requests are preferred; range-only servers receive a whole-document range request. Token colors override Tree-sitter where recognized; unsupported servers, unknown custom token types, missing tokens, and failed replies retain syntax highlighting. Text/path changes clear old semantic colors immediately, and stale replies cannot restore them. Same-text refreshes keep the current overlay until the replacement arrives.