perf: enable lazy compilation by default

[ScriptedAlchemy]

Summary

• enable guarded Rsbuild lazy compilation by default during development
• preserve explicit lazyCompilation: false and direct dev.lazyCompilation overrides
• add a changeset and update default-behavior coverage

Local verification

• pnpm test:core tests/index.test.ts failed before implementation with expected undefined to match object { entries: true, imports: true }
• pnpm test:core tests/index.test.ts tests/lazy-compilation.test.ts
• pnpm test:core
• pnpm build

Local benchmark signal

Compared against origin/codex/prewarm-route-workers with large dev profile, 3 iterations, 0 warmup:

Metric	Base	Head	Delta
Wall median	4.51s	5.04s	+11.7%

This PR is intentionally separate so CI can produce the authoritative 10-iteration benchmark comment before deciding whether to keep this default.

[github-actions]

Benchmark Results

Compared PR head 26447d5 against base 6c6000b.

Total median wall time: 9.53s -> 9.49s (-0.4%, 1.00x speedup)
Compiler ready median: 9.53s -> 9.49s (-0.4%)
Route load median: 1.98s -> 1.97s (-0.7%)

Benchmark	Base total	Head total	Delta	Head ready	Head routes	Speedup	Head RSS p95
large-355-ssr-esm	9.53s	9.49s	-0.4%	9.49s	1.97s	1.00x	-

Profile: large; mode: dev; iterations: 10; warmup: 0.
Workflow run