fix(herdr): resolve durable handle binding regression and reconcile stale slots (#242)

[James3014]

This PR fixes the HERDR durable handle binding regression and stale slot recovery on the public dispatch path:

1. Native path canonicalization and case-insensitive comparison on macOS (preventing false N4 Wrong Worktree rejections between /Users/james/Workspace and /Users/james/workspace).
2. Proper pre-launch error persistence: failExternalRuntimePreLaunchCAS now handles FENCED state without silently swallowing early failures.
3. Enhanced stale HerdR session reconciliation: supports reclaiming lost-handle sessions in FENCED and OUTCOME_UNKNOWN states after proving external absence in HerdR and inspecting clean physical workspace state.
4. Eager stale session reclamation before agent preflight and start.