Add cross-file call linker for C/C++/Go directory parsing

[Tomer-PL]

Summary

• Adds a post-merge _link_cross_file_calls() pass in parse_directory() that rewrites dangling CALLS edges to point at definitions in other modules
• Per-file parsers resolve bare calls like foo() as current_module:foo — after merging, if foo is defined in another module, that edge target doesn't exist and gets silently dropped by GraphStore._build_index
• The linker builds a name→node-id index of all functions/methods, then rewrites unresolved call edges by matching on bare function name

How it works

1. After all per-file graphs are merged, scan for CALLS edges whose target_id doesn't exist in the node set
2. Extract the bare function name (after the last : separator)
3. Skip qualified calls (::, ->, . in the bare name) — these are method calls that shouldn't be rewritten
4. If exactly one definition exists across all modules → rewrite with CERTAIN confidence
5. If multiple definitions exist → prefer the cross-module candidate (not in the caller's own module); if still ambiguous → pick the first with UNCERTAIN confidence

Impact

Tested on libavif v1.4.1 (24K lines of C, 818 nodes, 6,339 call edges):

Metric	Before	After
Resolved edges	1,099	2,780
Cross-module edges	0	1,681
Decode-path reachability	125 functions (1 module)	240 functions (15 modules)
Unresolved call targets	1,015	~380 (system/library calls)

Previously, paths_between("avifDecoderRead", "avifImageRGBToYUV") returned nothing because the call crosses from read.c to reformat.c. After this fix, the full decode→reformat→stream call chain resolves.

Test plan

• 3 new tests in test_common_parser.py covering unique resolution, same-module preservation, and unresolvable calls
• All existing tests pass (1,041 total)
• Validated on real-world C codebase (libavif)

Closes #10

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