test(dwarf): independent-parser gate via llvm-dwarfdump on emitted DWARF (#394)

[avrabe]

Summary

.debug_line + .debug_info emission for #394 (VCR-DBG-001) is already implemented and merged on main (PRs #429, #430, #443, #550, #557, #565, #573): synth emits a real line-number program plus a compile-unit DIE and one DW_TAG_subprogram DIE per function, all relocated against __synth_text_base, gated by 9 passing oracle tests (A–H).

The one deliverable #394's gate spec named that was still unmet: an independent-parser check. Oracles A–H parse synth's emitted DWARF with gimli::read — the same library gimli::write produced the bytes with — so a self-consistent emitter bug (an encoding gimli round-trips but real toolchains reject) would pass them. This PR adds that missing gate.

What's new — Oracle I (emitted_dwarf_verifies_with_llvm_dwarfdump_394)

Runs the independent LLVM DWARF parser against synth's emitted .o:

1. llvm-dwarfdump --verify must report "No errors" — its structural checks and the parent/child PC-range containment rule (the DWARF regression: compile_unit DW_AT_high_pc (line-table extent) doesn't cover child subprogram DIEs (code extent) — fails llvm-dwarfdump --verify post-link #564 / Oracle-H property, now cross-validated by a second implementation).
2. --debug-info must decode the CU + ≥1 DW_TAG_subprogram carrying DW_AT_name + DW_AT_low_pc/DW_AT_high_pc.
3. --debug-line must decode ≥1 line row.

Non-vacuous by construction: the tool is REQUIRED — the gate fails, never skips, when no llvm-dwarfdump is found. CI wires apt-get install -y llvm into the Test job so the binary is always present on ubuntu-latest. Locally the finder also accepts $LLVM_DWARFDUMP and the macOS Xcode-toolchain path.

Evidence — msgq_put_359 fixture

Verifying .debug_info Unit Header Chain...
Verifying unit: 1 / 1, "panicking.rs"
Verifying .debug_line...
No errors.

llvm-dwarfdump --debug-info (CU + 5 subprogram DIEs, real names):

DW_TAG_compile_unit   DW_AT_name ("panicking.rs")  low_pc 0x0  high_pc 0x42e
  DW_TAG_subprogram   DW_AT_name ("z_impl_k_msgq_put")                          [0x000,0x216)
  DW_TAG_subprogram   DW_AT_name ("core::panicking::panic_const::…add_overflow…") [0x218,0x22e)
  DW_TAG_subprogram   DW_AT_name ("gale::msgq::put_decide::h36409b4bbfd25a9a")   [0x230,0x362)
  DW_TAG_subprogram   DW_AT_name ("gale_k_msgq_put_decide")                      [0x364,0x426)
  DW_TAG_subprogram   DW_AT_name ("core::panicking::panic_fmt::h6651313c3e2c6c2f") [0x428,0x42e)

--debug-line (sample rows):

Address            Line  File
0x…0220            175   1
0x…0224            175   1
0x…0248            175   1

Oracle I summary line: --verify: No errors; --debug-info decoded 5 subprogram DIE(s); --debug-line decoded 109 row(s).

Test / CI

• cargo test -p synth-cli --test dwarf_debug_line_emit_394 — 10 passed (9 existing + Oracle I).
• cargo fmt --check, cargo clippy -p synth-cli --all-targets -- -D warnings clean.
• CI Test job (ubuntu-latest) now installs llvm and runs the gate via cargo test --workspace.

Refs #394 (VCR-DBG-001), #242.

🤖 Generated with Claude Code

https://claude.ai/code/session_01YJK5LZZEkV5smCY1jKn18L