revert(stdlib): narrow E2 to the 0105 type/alias close; defer value consts to E5 [stdlib E2 attempt-6]

Scope-narrowing revert of the value-const same-name sub-area (attempts 3-5),
per PO/Agra ruling. The 0105 type/alias close (per-source nominal struct
identity, source-keyed type aliases, F1 self/mutual refs, anon-struct
regression) is kept intact; cross-module same-name VALUE consts move to step E5.

- imports.zig: narrow `isPerSourceDecl` so a `const_decl` is retained
  per-source ONLY when its RHS introduces a TYPE (alias / inline type decl).
  VALUE consts (literal / value-expression RHS) and functions keep the pre-E2
  first-wins name-merge. Restores value-const reads to exactly the
  wt-stdlib-base (pre-E2) first-wins behavior.
- lower.zig / program_index.zig: restored to the pre-value-const state
  (66d10c0) — removes selectModuleConst / SourceConstCtx / pinConstAuthorSource
  / SelectedConst and the rewired comptimeIntNamed / float / runtime /
  global-init const reads; value-const reads return to the global path.
- examples: drop 0759-0762 (value-const own-wins / ambiguous / expr-chain-dim
  / leaf-author-pin) — they move to E5.

Kept green: 0752-0758 (same-name structs distinct + own-wins + ambiguous + self
/mutual ref), 0756 (alias per-source), 0170 (anon-struct field distinct).

Gate: zig build + zig build test (423/423, LSP sweep 513 no-crash) +
run_examples (496/0, prior markers byte-identical) + m3te ios-sim build exit 0.

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

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

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

@@ -1 +0,0 @@
-

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

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

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

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

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

@@ -1,5 +0,0 @@
-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

@@ -1 +0,0 @@
-

examples/expected/0761-modules-same-name-const-expr-chain-dim.exit

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

examples/expected/0761-modules-same-name-const-expr-chain-dim.stderr

@@ -1 +0,0 @@
-

examples/expected/0761-modules-same-name-const-expr-chain-dim.stdout

@@ -1 +0,0 @@
-a_len=2 a_val=2 b_len=11 b_val=11

examples/expected/0762-modules-same-name-const-leaf-author-pin.exit

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

examples/expected/0762-modules-same-name-const-leaf-author-pin.stderr

@@ -1 +0,0 @@
-

examples/expected/0762-modules-same-name-const-leaf-author-pin.stdout

@@ -1 +0,0 @@
-val=2 len=2