feat(rewriter): instruction-level offset map (#143 DWARF Phase 2 inc 2)

[avrabe]

Summary

Second increment of #143 DWARF Phase 2. meld's rewriter changes operand values (function/global/table/memory/type indices) whose LEB128 encodings shift byte length, so intra-function byte offsets drift during rewriting — .debug_line programs can't be remapped by function-base relocation alone. This adds the instruction-level old→new offset map that captures that drift.

Changes

• InstrOffset { old, new } + InstrOffsetMap { entries } with translate(old) -> Option<new>
• rewrite_function_body_with_offsets — parallel entry point returning (Function, InstrOffsetMap), sharing a private core with the existing rewrite_function_body (unchanged signature, zero caller churn in merger.rs)
• OLD positions from reader.into_iter_with_offsets(); NEW positions from per-instruction encoded-length measurement (identical bytes to Function::instruction, both via wasm_encoder::Encode)
• opt-in: the plain path pays nothing and emits byte-identical code

Why offsets are relative to the instruction stream

Both old and new are relative to the start of the function body's instruction stream (the byte after the locals vector). Increment 3 composes these intra-function offsets with the per-function base from the v0.16.0 component-provenance v2 code_range to translate DWARF code addresses input→fused.

Tests (4 new)

• instr_offset_map_tracks_leb_growth_from_index_remap — call 0→call 200 grows the operand LEB 1→2 bytes; divergence accumulates exactly [0,1,1,2,2,2] across call/drop/call/drop/const/end
• instr_offset_map_is_identity_when_no_leb_length_change — 0→1 keeps a 1-byte LEB; new == old everywhere
• instr_offset_map_translate_hits_and_misses
• with_offsets_emits_identical_function_bytes — offset collection must not perturb emitted code

No new LS-N

Pure infrastructure; output is byte-identical (proven). The wrong-DWARF-address hazard materializes only when increment 3 consumes this map — its LS-N lands there.

Test plan

• 4 new rewriter tests green
• 295 lib tests green, clippy + fmt clean
• CI green + Mythos AI scan on rewriter.rs (Tier-5)

🤖 Generated with Claude Code

[github-actions]

Mythos delta-pass required

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

meld-core/src/rewriter.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]

LS-N verification gate

⚠️ 35/37 verified — 2 missing regression tests

	count
Passed (≥1 test, all green)	35
Failed (≥1 test failure)	0
Missing (no ls_*_NN_* test found)	2

_{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

• LS-R-13
• LS-M-6

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

[github-actions]

Mythos delta-pass (auto)

✅ NO FINDINGS across 1 Tier-5 file(s)

File	Verdict	Hypothesis
``	✅ 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.}