fix: flow-chain + mermaid rendering improvements

[hungpham10]

Summary

Complete overhaul of call-chain extraction and Mermaid flow-chart rendering to show clear structure:

Extractor improvements

• Added markers: (17), (18), (20), (19), (21)
• handler tags innermost call with +
• handler tags innermost call with + (guarded by check)
• Fixed switch discriminant extraction for Rust (scan children instead of field 'condition')

Mermaid rendering

• Filter // from node rendering (group nested calls)
• Remap edges from original chain indices to filtered indices
• Run on filtered chain preserving IF/LOOP/SWITCH control flow
• Cross-pass dedup to eliminate duplicate edges (fixes bug where edges appeared twice)
• Call label appends when present
• Multiple expressions render as separate blocks

Tests

• Updated 21 golden chain tests to new marker format
• Added 5 new tests: , , , ,
• All 44 chain tests + 11 mermaid tests + 10 API tests pass

Verified via MCP

• (node 1535): 8 calls show (symbols, chains_raw, call_names_raw, self.files, etc.)
• (node 1047): 3 expressions render as separate switch blocks with , ,
• Charts are fully connected single flow (no scattered sub-charts)
• No more 'MARKER' nodes in output

[coderabbitai]

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[codspeed]

Merging this PR will degrade performance by 13.2%

⚠️ Different runtime environments detected

Some benchmarks with significant performance changes were compared across different runtime environments,
which may affect the accuracy of the results.

Open the report in CodSpeed to investigate

❌ 2 regressed benchmarks
✅ 36 untouched benchmarks

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Benchmark	BASE	HEAD	Efficiency
❌	open+ingest	252.9 ms	296.5 ms	-14.68%
❌	warm_no_hit	545.6 µs	617.8 µs	-11.69%

Tip

Investigate this regression by commenting @codspeedbot fix this regression on this PR, or directly use the CodSpeed MCP with your agent.

---

_{Comparing fix/flow-chain-mermaid-v3 (4180f84) with main (2bedb4e)}

[codecov-commenter]

Codecov Report

❌ Patch coverage is 94.96403% with 14 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.11%. Comparing base (2bedb4e) to head (4180f84).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/codegraph-extract/src/languages/common.rs	90.54%	7 Missing ⚠️
crates/codegraph-api/src/mermaid.rs	97.51%	4 Missing ⚠️
crates/codegraph-bench/src/bin/mem.rs	0.00%	1 Missing ⚠️
crates/codegraph-core/src/semgraph.rs	90.00%	1 Missing ⚠️
crates/codegraph-extract/src/languages/csharp.rs	0.00%	1 Missing ⚠️

❌ Your patch status has failed because the patch coverage (94.96%) is below the target coverage (100.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #45      +/-   ##
==========================================
+ Coverage   77.93%   78.11%   +0.17%     
==========================================
  Files          96       96              
  Lines       23067    23299     +232     
==========================================
+ Hits        17978    18200     +222     
- Misses       5089     5099      +10     

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