feat: materialize nonzero-sentinel compile-time byte slices

Own nonzero trailing sentinels in canonical string-backed snapshots and preserve specialization identity across computed and direct views.

Verified: bro build; bro test (296 passed); python3 tests/native.py (1026 interpreter/native comparisons at -O0/-O2, 5 native allocator programs, LLVM output and CLI errors).

README.md

@@ -136,7 +136,7 @@ ordinary procedures may compute integer, boolean, float and string compile-time
 
 computed callbacks materialize as references to their original source declarations, never as temporary interpreter function IDs. `choose proc() @proc(value i16) i16 { return increment }` can supply `$(choose())`, compile-time arguments and generated defaults. optional callbacks preserve absence and nested presence; arrays, records, tagged payloads and both fallible channels retain exact callback signatures and specialization-cache identity. imported private targets remain bound to their originating declarations, including across repeated compilation and native emission.
 
-readonly byte-slice results (`[]u8` and zero-terminated `[;0]u8`) snapshot their bytes into owned literals before evaluation storage is released. ordinary compile-time helpers can return string subviews, views of locally filled arrays or immutable global arrays, including nested records/arrays, tagged payloads, optionals and both fallible channels. explicit `$()` expressions, generic arguments and generated defaults preserve embedded NULs, arbitrary byte values, exact declared slice types and content-based specialization-cache identity. snapshots survive evaluation/session disposal and repeated compilation; uninitialized source bytes diagnose read-before-initialization. mutable slices, nonbyte or nominal-byte slices and nonzero-sentinel slices remain unsupported.
+readonly byte-slice results (`[]u8` and `[;sentinel]u8` for any byte sentinel) snapshot their bytes into owned literals before evaluation storage is released. ordinary compile-time helpers can return string subviews, views of locally filled arrays or immutable global arrays, including nested records/arrays, tagged payloads, optionals and both fallible channels. explicit `$()` expressions, generic arguments and generated defaults preserve embedded NULs, arbitrary byte values, exact declared slice types and content-based specialization-cache identity. nonzero-sentinel snapshots retain an owned trailing sentinel outside the logical length; empty views also retain it. snapshots survive evaluation/session disposal and repeated compilation; uninitialized source bytes diagnose read-before-initialization, and sentinel bytes are rechecked after evaluation. mutable slices and nonbyte or nominal-byte slices remain unsupported.
 
 `$(expression)` explicitly evaluates a constant expression, including an ordinary procedure call, even in a package without templates. use a typed expression when its shape cannot be inferred independently, such as `$(Point { x = 40 })`; integer literals retain contextual typing. the resulting value works in runtime expressions, bindings, expression statements and array lengths, while the erased expression still receives ordinary checking. each interpreter evaluation is isolated and limited to 100,000 IR instructions/terminators and 128 active frames; failures retain their originating source locations. runtime parameters and mutable local/global dependencies cannot supply compile-time inputs. nonnull data pointers, unsupported slice types and untagged unions cannot escape temporary evaluation as generated constants; active payloads or aggregate fields containing unsupported values are diagnosed rather than retaining interpreter handles. untagged unions lack an active-member identity, so selecting a source variant could discard observable bytes. executed foreign calls remain interpreter traps. general type-valued execution, broader storage-bearing materialization and remaining existing-library acceptance, including broader `meta` operations and `io.print`, remain open.