fix(lower): source-aware value-const resolution (own-wins / ambiguous) — close F2 [stdlib E2 attempt-3]

E2 retained per-source const declarations but left the const READ path on the
global last-wins `module_const_map`, so a module's OWN reference to a same-name
const bound the LAST global author (F2: a.sx `K::1`, b.sx `K::2`, main flat-imports
both → both read B's K). Complete the const analog of the type (`selectNominalLeaf`)
and callable (`selectPlainCallableAuthor`) source-aware models.

- `selectModuleConst`: own-wins; exactly one flat-visible author → it; ≥2 distinct
  flat-visible → `.ambiguous` (loud diagnostic, consistent with 0755/0724); none
  → `.none`. Reads the SELECTED author's per-source value (`module_consts_by_source`)
  and folds its RHS over the global leaf map, so a const-EXPRESSION chain
  (`N :: M + 1`, M flat-imported) still resolves M.
- Rewire `comptimeIntNamed` / `lookupFloatName` / `nameIsFloatTyped`, the runtime
  identifier path, and the global-init-from-const path through it; drop the now
  subsumed `moduleConstBareInvisible` gate.
- program_index: `moduleConst{Int,Float,IsFloatTyped}With` fold a selected `ci`.

Examples: 0759 (own-wins value const, a=1 b=2) + 0760 (two-flat-visible →
ambiguous). Single-author byte-identical (run_examples 498/0, 496 prior unchanged;
zig build test 423/423; corpus sweep 515 no-crash; m3te ios-sim exit 0).

examples/expected/0759-modules-same-name-const-own.exit

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

examples/expected/0759-modules-same-name-const-own.stderr

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+

examples/expected/0759-modules-same-name-const-own.stdout

@@ -0,0 +1 @@
+a=1 b=2

examples/expected/0760-modules-same-name-const-ambiguous.exit

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+1

examples/expected/0760-modules-same-name-const-ambiguous.stderr

@@ -0,0 +1,5 @@
+error: 'K' is ambiguous: it is declared in multiple flat-imported modules; qualify the reference or remove the duplicate import
+  --> examples/0760-modules-same-name-const-ambiguous.sx:12:21
+   |
+12 |     print("K={}\n", K);
+   |                     ^

examples/expected/0760-modules-same-name-const-ambiguous.stdout

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+