perf(build): expose postprocess stage timings

[tirth8205]

Summary

• add structured postprocess_timing data to build results
• keep the minimal-mode schema to signatures_s and fts_s
• add flows_s, communities_s, and summaries_s in full mode
• keep postprocess none unchanged and emit no extra stdout or timing logs

Attribution

This ports only the safe newest timing commit from #336 by @gzenz onto the current main pipeline. Gideon Zenz is preserved as a co-author.

Verification

• focused postprocess tests after rebasing onto protected main 8698ce2: 3 passed
• full test suite after rebasing onto protected main 8698ce2: 1507 passed, 1 skipped, 2 xpassed
• Ruff: code_review_graph/tools/build.py passed
• test-file E/F lint passed
• graph review: 0 affected execution flows; timing paths covered by full and minimal regression tests

[github-actions]

code-review-graph review

Overall risk: 0.60 (MEDIUM) — 4 changed function(s)/class(es), 0 affected flow(s), 2 test gap(s)

Risk-scored changes

Risk	Level	Symbol	Location	Tested
0.60	medium	code_review_graph/tools/build.py::_run_postprocess	code_review_graph/tools/build.py:16	no
0.30	low	tests/test_tools.py::TestBuildPostprocess	tests/test_tools.py:1274	no
0.30	low	tests/test_tools.py::TestBuildPostprocess.test_postprocess_minimal_has_fts_no_flows	tests/test_tools.py:1309	(test)
0.30	low	tests/test_tools.py::TestBuildPostprocess.test_postprocess_full_matches_default	tests/test_tools.py:1335	(test)

Test gaps

• code_review_graph/tools/build.py::_run_postprocess (code_review_graph/tools/build.py:16)
• tests/test_tools.py::TestBuildPostprocess (tests/test_tools.py:1274)

Token savings: this graph-backed report used ~21,064 fewer tokens (~95%) than reading every changed file in full (estimated, chars/4 approximation).

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