docs(website): document the run-result format as a stable contract

[AndreasKoestler]

Summary

Closes #66.

Adds a reference page, Run results, documenting the run-result format as a stable, machine-readable contract:

• File layout — .var/<spec>.json, one record per spec, git-ignored, written by the vitest reporter
• Record shape — SpecResults / ExampleResult / CellFailure with annotated passing + failing examples; expected recovered by slicing source at the recorded offsets; ReturnShapeError/plain errors carry message+stack only
• sourceHash — FNV-1a 32-bit over UTF-16 code units, fnv1a: prefix, pinned test vectors for consumer reimplementation, and the staleness contract (hash mismatch → consumer must not render)
• Drift baseline — var.lock.json shape, byte-stable serialization, acknowledgment flows (var run --update, VAR_UPDATE=1, LSP var.acceptDrift), vitest as read-only gate
• Stability policy — additive optional fields without a version bump; breaking changes bump version; consumers must ignore unknown fields and treat unknown versions as absent; the hash algorithm is versioned independently via its prefix. Aimed at long-lived consumers (supervising agents, CI gates, CRA attestation pipelines)
• Cross-language rules — UTF-16 offsets, POSIX specPath, 2-space JSON + trailing newline; per-port emitter status table (TS today; py/java/ruby/rust planned)

Drive-bys: CLAUDE.md now points at the stable doc instead of calling the format "emerging"; ADR 0002's dead examples-and-drift.mdx links fixed (examples.mdx).

Notes

• The issue comment mentioned varar.lock.json — the actual filename is var.lock.json; the page documents the real name.
• Follow-up issues filed separately: var run --json reporter, and run-record emitters for the Python/Java/Ruby/Rust ports.

Verification

• pnpm --filter @oselvar/website build — content sync accepts the new page (build then fails on a pre-existing TabItem/unified prerender error on how-to/tables-and-doc-strings, unrelated to this change)
• All field names, hash vectors, CLI flags, and LSP command names checked against var-core/src/{result,hash,drift,failure,run-diagnostics}.ts, var-vitest/src/reporter.ts, var-cli/src/run.ts, and var-lsp/src/store.ts