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/0153-types-backtick-const-fn-decl.sx

@@ -0,0 +1,23 @@
+// Backtick raw-identifier escape at the `::` declaration sites: a leading
+// backtick makes a CONSTANT name and a FUNCTION name raw, so a reserved type
+// spelling (`s2`, `u8`) can be declared and used. Complements examples/0151
+// (var / param / field / global). The backtick fn is callable both via the
+// backtick (`` `u8(5) ``) and bare (`u8(5)`) — the bare reserved-name callee
+// resolves to the raw fn because its declaration is raw (issue 0089). A *bare*
+// `s2 :: …` / `u8 :: …` declaration is still the reserved-name error (see
+// examples/1140).
+// Regression (issue 0089).
+#import "modules/std.sx";
+
+// Constant whose name is a reserved type spelling.
+`s2 :: 2.5;
+
+// Function whose name is a reserved type spelling.
+`u8 :: (n: s64) -> s64 { return n + 7; }
+
+main :: () -> s32 {
+    print("const    = {}\n", `s2);
+    print("fn tick  = {}\n", `u8(5));
+    print("fn bare  = {}\n", u8(5));
+    return 0;
+}

examples/1140-diagnostics-reserved-name-const-fn-decl.sx

@@ -0,0 +1,19 @@
+// A reserved/builtin type-name spelling is rejected as the NAME of a `::`
+// declaration too — both a constant (`s2 :: 5`) and a function
+// (`u8 :: (…) {…}`). A function name and a const name are binding sites just
+// like `s2 := …`; previously the `::` decl forms slipped past the
+// reserved-name check, so a bare reserved-name function compiled silently and
+// became callable — bypassing the backtick rule that handwritten sx must use.
+// The backtick escape (`` `s2 :: … ``, examples/0153) is the only way to spell
+// these names; `#import c` foreign decls remain exempt (examples/1220).
+//
+// Regression (issue 0089). Expected: one error per declaration, each caret on
+// the declared name; exit 1.
+#import "modules/std.sx";
+
+s2 :: 5;
+u8 :: (n: s64) -> s64 { return n + 7; }
+
+main :: () -> s32 {
+    return 0;
+}

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 @@
+