DWARF Phase 2: address remap (#130)

[avrabe]

Sub-issue of #130 — Phase 2 of the witness-mapping epic.

v0.7 shipped Phase 1.5 (`DwarfHandling::Strip` default — stop being wrong). Phase 2 is the real fix: rewrite DWARF addresses across the merged code section so downstream MC/DC tooling (`pulseengine/witness`) can produce correct source-line attribution on fused output.

Scope

Per #130's plan §"Phase 2 — DWARF remap":

• `.debug_line` rewrite. Single-pass line-program rewrite using the function-body relocation map the merger already builds. Addresses advance sequentially and meld preserves per-body byte offsets modulo index reencoding — straightforward.
• `.debug_info` rewrite. DIEs that reference code addresses (`DW_AT_low_pc`, `DW_AT_high_pc`, `DW_AT_ranges`) get the same remap. CU concatenation with offset adjustments for multi-input cases.
• `.debug_ranges` / `.debug_rnglists`. Entry-by-entry rewriting.
• `.debug_str` / `.debug_abbrev` passthrough. Address-free; `.debug_str` needs string-pool dedup across inputs; `.debug_abbrev` merges if byte-equal, else stays per-CU.
• New `DwarfHandling::Remap` variant on `FuserConfig` — opt-in initially, flipped to default once verified against witness on real-world fused modules.
• End-to-end fixture: a fused module from `tests/wit_bindgen/fixtures/` driven through witness's gimli-based `(code-offset → file, line)` build path; assert >X% of `br_if` byte offsets get source attribution.

Dependencies

• Add `gimli` (read) + a writer path to `meld-core`. Forking the writer-side logic into a thin standalone helper is OK if pulling the full gimli writer pulls too many deps.

Why not in v0.8 / v0.9

Real DWARF surgery is multi-day work. Witness integration is the verifier — needs the cross-repo loop set up. v0.10 is the soonest realistic milestone.

Out of scope

• Synthesised DIEs for adapter / inlined code (that's Phase 3, #TBD)
• Variable-level debug info (`.debug_loc`, `.debug_loclists`) — explicitly out per DWARF / source-map remapping for fused output (witness MC/DC integration) #130.

Rivet artifact

• New requirement SR-35 — "DWARF address remap into merged code section".

Milestone

v0.10.0