Tombstone replay edge removals

[TZZheng]

Fixes #564.

Summary

• emit explicit replay delta tombstones for removed avatar/contact/mail edges
• apply those edge removals in the portal frontend replay reconstruction path, including removal-only deltas
• version replay chunk caches as v:2
• preserve readable legacy v:0 replay chunks when JSONL history cannot fully reconstruct that hour, while rewriting to v:2 when JSONL coverage is complete

Why

Replay deltas already tombstone removed nodes with State: "__REMOVED__", and the frontend deletes those nodes during reconstruction. Edge deltas did not have an equivalent removal path: backend deltas only added/changed avatar/contact/mail edges, and the frontend only upserted edge arrays.

That meant a removed edge could persist in reconstructed replay frames until the next keyframe, up to the keyframe interval, and could temporarily dangle toward nodes that had already been tombstoned.

Behavior

• FrameDelta now has avatar_edges_removed, contact_edges_removed, and mail_removed tuple lists.
• computeDelta emits deterministic removed-edge tuples by comparing previous edge keys against current edge keys.
• reconstructFrames deletes removed edges using the same composite keys as the existing upsert paths, then applies add/change deltas.
• Removal-only deltas are applied correctly.
• Existing node tombstone behavior is unchanged.
• ReplayChunk now carries cache format version v:2.

Legacy cache handling

The cache migration is intentionally compatibility-preserving:

• valid v:2 chunks are served directly;
• readable legacy chunks with missing/stale version are distinguished from corrupt/unreadable cache files;
• if JSONL fully covers the cached hour, the chunk is re-encoded and rewritten as v:2;
• if JSONL is empty or only partially covers that historical hour, the readable legacy chunk is served as a compatibility fallback rather than blanking historical replay;
• corrupt/unreadable cache files are not served as compatibility fallback.

This preserves old replay history after topology JSONL compaction/truncation, while ensuring newly generated chunks use the removal-capable wire format.

Non-goals

• no change to node tombstone representation
• no change to keyframe interval or recording cadence
• no frontend test-runner infrastructure added
• no topology producer changes outside replay encoding/reconstruction

Validation

Parent and reviewer validation included:

• git diff --check canonical/main...HEAD
• cd portal && go test ./internal/api/... -count=1
• cd portal/web && npm ci && npm run build
• cd portal && go build ./...
• cd portal && go vet ./...
• cd portal && go test ./... -count=1

Focused tests cover:

• removed avatar edges
• removed contact/mail edges, including removal-only deltas
• node removal together with dependent edge removals
• delta encode/reconstruction contract for edge add/remove
• stale v:0 cache rewrite when JSONL coverage is complete
• stale v:0 compatibility serving when JSONL has no frames
• stale v:0 compatibility serving when JSONL coverage is partial
• corrupt cache rejection

npm ci reported existing audit advisories during local validation; dependency remediation is outside this issue.

Review

This went through the requested multi-review workflow:

• Codex review: APPROVE, no blockers/nits.
• Claude Code review: APPROVE-WITH-NITS, no blockers, public-PR-ready.
• GLM review: APPROVE, no blockers/nits.

The only review feedback was optional robustness cleanup for future consideration; no reviewer requested a blocking code change.