feat(lang): reserved-name check covers :: const/fn/type decls + scope call rewrite to raw provenance [F0.6]

A bare reserved-type-name `::` declaration was silently accepted, and the
attempt-2 lowerCall rewrite then made a bare `s2 :: (…) {…}` function callable —
bypassing the backtick rule for handwritten sx. The reserved-name binding check
covered `:=` / typed-local / param / captures but NOT the `::` declaration form.

- ast: `ConstDecl`/`FnDecl` carry `is_raw` + `name_span` threaded from the parser
  (parseConstBinding / parseFnDecl, all call sites incl. struct/impl methods).
- semantic_diagnostics: reject a bare reserved spelling at EVERY declaration-name
  site — const, function (incl. struct/impl methods), struct/enum/union/error-set,
  protocol, foreign-class, ufcs alias, namespaced/library/c-import name. Backtick
  (`is_raw`) and the compiler's `#builtin` definition (`string :: []u8 #builtin`)
  are the only exemptions; a value whose node is itself a named decl defers to
  that node's own check.
- c_import: synthesized foreign fn_decls are `is_raw = true`, so a C function
  whose own name collides with a reserved spelling (`int s2(int);`) imports and
  bare-calls unedited.
- lower: scope the `.type_expr`→`.identifier` call rewrite to a callee FnDecl of
  RAW provenance (`is_raw`) — only a backtick / `#import c` foreign fn can carry a
  reserved-name spelling, so a non-raw match never gets rewritten.
- examples: 0153 (positive — backtick `::` const + fn, bare + tick call), 1140
  (negative — bare `::` const + fn rejected).
- docs: specs.md + readme.md state the backtick is required at every binding site
  including `::` const / function / type declarations; issue 0089 banner updated.

issues/0089-backtick-raw-identifier.md

@@ -16,6 +16,17 @@
 >    ([src/ir/semantic_diagnostics.zig]). The backtick works in every identifier
 >    position (local, global, param, field, function name, struct member, later
 >    reference, and all the control-flow/capture/binding forms).
+>
+>    The `::` DECLARATION forms are binding sites too and are equally covered
+>    (F0.6 attempt-3): a bare reserved-name **constant** (`s2 :: 5`), **function**
+>    (`s2 :: (…) {…}`, incl. struct/impl methods), or **type** declaration
+>    (`struct`/`enum`/`union`/`error`/alias/`protocol`/foreign-class/ufcs/namespace)
+>    is rejected, exactly like `s2 := …`. `ConstDecl`/`FnDecl` carry `is_raw` +
+>    `name_span` threaded from the parser (`parseConstBinding`/`parseFnDecl`), so the
+>    backtick form (`` `s2 :: … ``) is exempt; the compiler's own builtin definition
+>    (`string :: []u8 #builtin`) is the sole non-backtick exemption (a `#builtin`
+>    constant defines the reserved type). This closed the attempt-2 hole where a
+>    bare `s2 :: (…) {…}` compiled silently and the call rewrite made it callable.
 > 2. **`#import c` foreign-name exemption.** `c_import.zig` synthesizes foreign
 >    `#foreign` decls with `Param.is_raw = true`, so generated C param names that
 >    collide with reserved type names (`s1`, `s2`) import unedited.
@@ -24,8 +35,11 @@
 > one in type position (`x : `s2 = 1`) is a clean parse error ([src/parser.zig]
 > `parseTypeExpr` atom). A reserved-spelled FUNCTION (backtick-declared or
 > `#import c` foreign) is bare-callable: `lowerCall` rewrites a `.type_expr` callee
-> to an identifier when a function of that name is in scope ([src/ir/lower.zig]),
-> so `s2(4)` resolves to the function (`TypeName(val)` is not a cast). A later BARE
+> to an identifier when a function **of RAW provenance** of that name is in scope
+> ([src/ir/lower.zig]) — the rewrite is scoped to the callee `FnDecl`'s `is_raw`
+> flag (F0.6 attempt-3), so it only ever fires for a backtick / `#import c` foreign
+> fn (the decl check guarantees no bare reserved-name fn exists), so `s2(4)`
+> resolves to the function (`TypeName(val)` is not a cast). A later BARE
 > reference in value position resolves to the binding; a bare `s2` in type position
 > is still the type.
 >
@@ -38,7 +52,11 @@
 > (foreign param + function-name exemption, bare-callable foreign fn),
 > `examples/1139-diagnostics-backtick-raw-not-a-type.sx` (negative — raw in type
 > position), `examples/1119`/`1121`/`1123` (negative — bare reserved binding still
-> rejected across all forms). Backtick lexer unit tests in `src/lexer.zig`.
+> rejected across all forms),
+> `examples/0153-types-backtick-const-fn-decl.sx` (positive — backtick `::` const +
+> function decl, bare + backtick call), and
+> `examples/1140-diagnostics-reserved-name-const-fn-decl.sx` (negative — bare `::`
+> const + function decl rejected). Backtick lexer unit tests in `src/lexer.zig`.
 >
 > The original report is preserved below.