feat(lang): backtick raw-identifier escape + #import c foreign-name exemption [F0.6]

Reserved type-name spellings (s1, s2, u8, …) can now be used as value
identifiers two ways, resolving issue 0089:

1. Backtick raw identifier: a leading backtick (`s2) lexes to an
   .identifier token carrying a new Token.is_raw flag, with the backtick
   excluded from the text. A raw identifier is never type-classified — the
   parser skips Type.fromName for it — so it is always a value identifier.
   The flag threads to VarDecl.is_raw / Param.is_raw at binding sites, and
   the reserved-type-name check (UnknownTypeChecker) skips raw bindings.
   Because the token tag stays .identifier, the escape works in every
   position (local, global, param, field, fn name, struct member, later
   reference) with no per-site parser change.

2. #import c exemption: c_import.zig synthesizes foreign decls with
   Param.is_raw = true, so generated C param names that collide with
   reserved type names (s1, s2) import unedited.

A bare reserved-name binding in sx still errors (issue 0076 preserved):
the is_raw-gated skip only fires for backtick / foreign names, and a raw
binding's address-of / autoref lowering stays correct because every
occurrence is an .identifier, never a .type_expr.

Tests: examples/0151 (backtick, every position),
examples/1220 (foreign exemption, compiled+run), lexer unit tests.
1119 (bare-binding rejection) stays green. specs.md + readme.md updated.

readme.md

@@ -105,6 +105,19 @@ y : s32 = 0;           // explicit type
 z : s32 = ---;         // uninitialized
 ```
 
+Builtin type names (`s2`, `u8`, `bool`, `string`, …) are reserved and can't be used
+as bare value identifiers. A leading backtick escapes one into a raw identifier — its
+text drops the backtick and it's never read as a type — so reserved spellings (and
+keywords) work as ordinary names:
+
+```sx
+`s2 := 2.5;            // value identifier "s2", distinct from the s2 type
+print("{}\n", `s2);    // 2.5
+```
+
+Foreign declarations from `#import c { … }` are exempt automatically: C names that
+collide with reserved type names (e.g. `s1`, `s2`) import unedited.
+
 ### Structs
 
 ```sx