feat(adr7): static-PIC data/element-offset fold — inc 4 (#353)

[avrabe]

Implements ADR-7 path-H, increment 4 — static-PIC data/element-offset folding
(#353) — reproduced and fixed in one branch.

The bug (reproduced)

A data/element segment offset const-expr may global.get only an imported
global. In a PIC dylib, (data (global.get $__memory_base) …) is valid because
__memory_base is imported. But when meld fuses a shared-everything graph, a
$main module provides __memory_base, so after merge it becomes a defined
global — and meld emitted (data (global.get $base) …) verbatim. wasm-tools
validates it (lenient); wasmtime rejects it at instantiation ("constant
expression required: global.get of locally defined global") → silent invalid
output from valid input.

(The spike's original "2 memories / new topology modeling" premise did not
reproduce on current meld — see the earlier #353 comments; the real residual was
this offset fold.)

The fix

Fold a global.get of a defined constant-i32 global to i32.const <value>
in offset emission. Imported globals stay a verbatim global.get, preserving
#338.

• segments.rs — const_i32_init_value extracts a global init's constant
  i32; ParsedConstExpr::reindex folds GlobalGet(defined-const) → I32Const.
  This sits in reindex, so both data and element offsets are covered.
• merger.rs — MergedModule.defined_global_i32_const records defined
  constant-i32 globals at global-merge; copied into IndexMaps for the reindex.
• rewriter.rs — IndexMaps.defined_global_i32_consts (empty default → no
  fold, so every other caller is unaffected).

Verification

• Execution oracle pic_base_relative_data_reads_correctly_after_fold — a
  hand-written PIC-pattern component (base-relative data + a lifted read) now
  fuses, validates, and executes on wasmtime with the data read back
  correctly (0xddccbbaa) at the folded base. A wrong fold (value/index/address)
  fails this.
• meld fuse silently truncates extended-const exprs beginning with global.get (drops trailing arithmetic) — corrupts global initializers + data/element offsets (PIE base+N) #338 regression — const_expr_globalget (4 tests, imported-base) still
  green: imported globals stay verbatim.
• Structural baseline (shared_everything_fuses_to_valid_single_memory_core)
  against a real wasm-tools component link fixture still green.
• Full meld-core suite green (0 failures); fmt + clippy clean.

Tier-5 (merger/segments/adapter) → Mythos delta-pass below.

Refs #353 (ADR-7 path-H inc 4), #338

[github-actions]

LS-N verification gate

✅ 59/59 approved LS entries verified

	count
Passed (≥1 test, all green)	59
Failed (≥1 test failure)	0
Missing (no ls_*_NN_* test found)	0

_{Approved loss-scenarios.yaml entries are expected to have a
regression test named ls_<letter>_<num>_* (e.g. LS-A-11 →
ls_a_11_*). The gate runs each prefix via cargo test --lib --no-fail-fast and aggregates pass/fail/missing.}

Failed LS entries

(none)

Missing regression tests

(none)

_{Updated automatically by tools/post_verification_comment.py.
Source of truth: safety/stpa/loss-scenarios.yaml.}

[github-actions]

Mythos delta-pass required

This PR modifies one or more Tier-5 source files (per
scripts/mythos/rank.md):

meld-core/src/adapter/fact.rs
meld-core/src/merger.rs
meld-core/src/rewriter.rs
meld-core/src/segments.rs

Before merge, run the Mythos discover protocol on the
modified Tier-5 files:

1. Follow scripts/mythos/discover.md
   — one fresh agent session per touched Tier-5 file.
2. For each finding, the agent must produce both a Kani
   harness and a failing PoC test (per the protocol's
   "if you cannot produce both, do not report" rule).
3. Attach a comment on this PR with either the findings
   (formatted per discover.md's output schema) or
   NO FINDINGS.
4. Add the mythos-pass-done label to this PR.

Why this gate exists: LS-A-10
(CABI alignment padding in async-lift retptr writeback) was
found by the v0.8.0 pre-release Mythos pass — but it had
lived in the callback emitter since #128, across six
releases. A PR-time gate would have caught it at review
time instead of at the release boundary.

The gate check on this PR will pass once the label is
applied.

[github-actions]

Mythos delta-pass (auto)

✅ NO FINDINGS across 4 Tier-5 file(s)

File	Verdict	Hypothesis
``	✅ NO FINDINGS	—
``	✅ NO FINDINGS	—
``	✅ NO FINDINGS	—
``	✅ NO FINDINGS	—

_{Auto-run via anthropics/claude-code-action@v1
(SHA-pinned) on the touched Tier-5 files, using the
maintainer's Max-plan OAuth token. See
.github/workflows/mythos-auto.yml and
scripts/mythos/discover.md.}