Preserve DWARF line-zero rows

[cpunion]

Fixes #9164.

Problem and fix

Line 0 means “no corresponding source line,” not “dead address.” Dropping that row incorrectly extends the preceding source line over those instructions.

- return line != 0 && addr != 0;
+ return addr != 0;

Address-zero and tombstone-sequence filtering remain unchanged. The production change is one condition and its comment; regenerated snapshots are in a separate commit.

Before / after

Check	Before (main f3ab996)	After
Original #9164 repro: call at 0x0b	Incorrect line 5	Original line 0
TinyGo: roundtrip line-table coverage	94.14%	99.97%
TinyGo: O1 line-table coverage	94.02%	99.96%
TinyGo: Asyncify + O1 line-table coverage	82.18%	99.72%

Coverage here means code bytes covered by any line-table row, including line 0, within functions having DWARF—not source-test coverage. Both versions processed the same TinyGo goroutine/JSON input. Stripped Asyncify outputs are byte-identical.

Validation and scope

• Regression fails before the fix and passes after it. Covers positive/zero line transitions, all-zero sequences, end-sequence rows, address remapping, roundtripping, and dead-address sentinels.
• 92 Python tests (including 9 DWARF tests), the wasm-opt suite, LLVM DWARF verification, and runtime checks pass. C++: 437 passed, one existing platform skip. V8 execution checks were not run locally.

This only preserves existing rows. Encoding changes are in #9196; instruction-origin handling is in #9194. It does not generate helper DWARF or fully resolve tinygo-org/tinygo#5736.

Original contribution, developed with AI assistance.