feat(pi): session fencing, daemon session leases, diff renderer core

[Patel230]

Summary

Follow-up to the Pi adoption P0 PR (#226). Implements the P1 items (and the P2 #6 lease work) from docs/plans/pi-adoption-plan.md:

• Session writer fencing (internal/session): additive Fence metadata field persisted on save/load, plus FenceOf/SetFence and WAL AppendMetaWithFence. Backward-compatible.
• Daemon session leases (internal/daemon): POST /v1/sessions/{id}/lease (acquire, returns a fence) and DELETE .../lease?fence= (release, requires the matching fence). Single-owner enforcement with 409 on stale fence. OpenAPI updated (parity test green).
• Differential renderer core (internal/tui/diff): line-granular diff + synchronized-output emission (mirrors pi-tui), fully unit-tested. The full Bubble Tea render-path swap is a runtime-verified follow-up.
• Kitty graphics (P2) remains deferred pending the render-path integration.

Test plan

• Full go test ./... green (via pre-push hook + focused rerun)
• New tests: daemon lease acquire/release, session fence round-trip, diff renderer range/sync
• make vet, make lint, hawk verify green
• Boundary guards, submodule drift, govulncheck passed in pre-push hooks
• Rebased onto the merged P0 commit; PR diff contains only P1/P2