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.

examples/expected/0153-types-backtick-const-fn-decl.exit

@@ -0,0 +1 @@
+0

examples/expected/0153-types-backtick-const-fn-decl.stderr

@@ -0,0 +1 @@
+

examples/expected/0153-types-backtick-const-fn-decl.stdout

@@ -0,0 +1,3 @@
+const    = 2.500000
+fn tick  = 12
+fn bare  = 12

examples/expected/1140-diagnostics-reserved-name-const-fn-decl.exit

@@ -0,0 +1 @@
+1

examples/expected/1140-diagnostics-reserved-name-const-fn-decl.stderr

@@ -0,0 +1,11 @@
+error: 's2' is a reserved type name and cannot be used as an identifier
+  --> examples/1140-diagnostics-reserved-name-const-fn-decl.sx:14:1
+   |
+14 | s2 :: 5;
+   | ^^
+
+error: 'u8' is a reserved type name and cannot be used as an identifier
+  --> examples/1140-diagnostics-reserved-name-const-fn-decl.sx:15:1
+   |
+15 | u8 :: (n: s64) -> s64 { return n + 7; }
+   | ^^

examples/expected/1140-diagnostics-reserved-name-const-fn-decl.stdout

@@ -0,0 +1 @@
+