docs: update build performance benchmarks (3.1.2)

[github-actions]

Automated build benchmark update for 3.1.2 from workflow run #271.

[greptile-apps]

Greptile Summary

This is an automated documentation-only PR that appends build performance benchmark data for version 3.1.2 to generated/benchmarks/BUILD-BENCHMARKS.md and syncs the performance summary table in README.md with the latest measurements.

Key changes:

• New benchmark rows for 3.1.2 (native + WASM) added to all benchmark tables: per-file build speed, incremental rebuilds, and query benchmarks.
• Build-phase breakdown table expanded to include dedicated columns for 1-file rebuild timings alongside full-build timings.
• Raw totals updated to reflect 235 files (up from 180 in 3.1.0) with a native build time of 1.2 s and WASM at 3.5 s.
• 50k-file estimates updated (native: ~260 s, WASM: ~750 s), showing measurable improvement over 3.1.0 (previously 305 s / 825 s).
• README summary table updated to reflect the rounded 3.1.2 values (e.g. build speed native: 5.2 ms/file, 1-file rebuild: 296 ms).
• All values cross-check correctly against the embedded raw JSON data block (per-file figures, total build times, DB sizes, and estimated projections are internally consistent).

Confidence Score: 5/5

• This PR is safe to merge — it contains only automated documentation updates with no code changes.
• Both changed files are Markdown documentation. The benchmark numbers are internally consistent across the README summary, the BUILD-BENCHMARKS tables, and the embedded JSON data block. No source code, tests, or configuration files are touched.
• No files require special attention.

Important Files Changed

Filename	Overview
README.md	Updates the performance summary table with 3.1.2 metrics; all values match the corresponding benchmark data in BUILD-BENCHMARKS.md.
generated/benchmarks/BUILD-BENCHMARKS.md	Adds 3.1.2 benchmark rows (native + WASM), expands the build-phase table to include 1-file rebuild columns, appends incremental rebuild and query benchmark rows, and embeds the full raw JSON data block; values are internally consistent.

Sequence Diagram

sequenceDiagram
    participant CI as CI Workflow #271
    participant CG as codegraph (self-analysis)
    participant BM as BUILD-BENCHMARKS.md
    participant RD as README.md

    CI->>CG: Run build benchmark (native + WASM) on v3.1.2
    CG-->>CI: native: 5.2ms/file, query: 4.2ms, no-op: 6ms, 1-file: 296ms
    CG-->>CI: wasm: 15ms/file, query: 5.8ms, no-op: 7ms, 1-file: 563ms
    CI->>BM: Prepend new rows for 3.1.2 (native + wasm)
    CI->>BM: Update Raw Totals section (files: 235, build: 1.2s/3.5s)
    CI->>BM: Expand phase-breakdown table (add 1-file rebuild columns)
    CI->>BM: Append incremental rebuild rows (3.1.2 native + wasm)
    CI->>BM: Append query benchmark rows (3.1.2 native + wasm)
    CI->>BM: Embed raw JSON data block for 3.1.2
    CI->>RD: Update performance summary table with rounded 3.1.2 values
    CI->>CI: Open PR #406 with doc-only changes

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_{Last reviewed commit: 4a2e04b}