perf(core): batch planned action stability waits

[dexhunter]

Fixes #93

Summary

• wrap each planned action sequence in connector-level batch lifecycle handling
• coalesce ordinary browser stability waits at the batch boundary while preserving the largest requested timeout
• keep immediate barriers for explicit navigation and tab switches, plus a 500 ms navigation-detection window for click-driven transitions
• support nested batches and safely ignore service-worker and non-navigation requests

Performance

A deterministic seven-sample harness exercised the real Agent -> action resolver -> WebHarness path with three consecutive keyboard:type actions on the same stable page.

	Stability waits	Median batch time
main (f1b587c4)	3	3326.816 ms
this branch	1	2299.677 ms

That removes two redundant waits and reduced median batch time by 30.87% on the test host. The harness also verifies action order, arguments, results, emitted events, memory recording, and planning-call count.

Safety coverage

• delayed click navigation completes before the next action runs
• explicit navigation and tab switches retain immediate stability barriers
• pending timeout requirements are not discarded by an immediate barrier
• nested batches coalesce without deadlocking
• service-worker requests never call Request.frame()

Validation

• bun run check
• bun run build
• bun test packages/magnitude-core/src (40 passed)
• focused lifecycle test repeated five times (4 passed each run)
• external safety harness (8 assertions)
• git diff --check