feat(benchmark): add generic evaluation and comparison harness

[wuyuanfr]

Summary

• add a generic Langfuse-backed benchmark runner with evaluator, manifest, integrity, web-evidence, and replay support
• add paired P/C and cross-worktree Legacy/P comparison runners with score synchronization and report generation
• relocate Langfuse infrastructure and ctx_debugger under sdk/benchmark
• add standard benchmark configs, bilingual documentation, and benchmark regression tests
• remove the unused production parity snapshot public entrypoint and exporter

Scope

The final diff against develop is limited to benchmark code/tests and the expected sdk/ctx_debugger relocation cleanup. It does not change sdk/nexent/core relative to the current develop branch.

Validation

• test/sdk/benchmark: 122 passed
• generic benchmark and test Ruff: passed
• ctx_debugger and LongMemEval Ruff: passed
• git diff --check: passed

Real Langfuse smoke runs

• single: gaia-web-search-single-postmerge-ecd8477-v2 — 1/1, score 1.0, no trace error
• Legacy/P: gaia-web-search-lp-postmerge-ecd8477-v1 — PP=1, both scores 1.0, no trace error
• P/C: gaia-web-search-pc-postmerge-ecd8477-v1 — PP=1, both scores 1.0, manifest parity passed, C compression triggered 1/1

Evidence to add

• Langfuse single-run screenshot
• Legacy/P comparison screenshot
• P/C comparison and compression screenshot

[codecov]

Codecov Report

❌ Patch coverage is 85.47297% with 430 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...enchmark/generic/run_context_manager_comparison.py	77.72%	55 Missing and 35 partials ⚠️
sdk/benchmark/generic/run_legacy_p_comparison.py	74.19%	34 Missing and 22 partials ⚠️
sdk/benchmark/agent_runner.py	85.15%	11 Missing and 23 partials ⚠️
sdk/benchmark/generic/tools/web_evidence.py	78.87%	16 Missing and 14 partials ⚠️
...k/benchmark/generic/evaluators/gaia_exact_match.py	70.32%	17 Missing and 10 partials ⚠️
sdk/benchmark/generic/run_failed_item_repeats.py	81.20%	14 Missing and 11 partials ⚠️
sdk/benchmark/generic/run_benchmark.py	94.65%	9 Missing and 15 partials ⚠️
sdk/benchmark/generic/tools/export_agent_config.py	75.28%	14 Missing and 8 partials ⚠️
sdk/benchmark/generic/run_integrity.py	87.09%	13 Missing and 7 partials ⚠️
sdk/benchmark/generic/common/secret_refs.py	79.41%	11 Missing and 3 partials ⚠️
... and 10 more

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