lnrpc/chainrpc: surface re-org depth and Done over the chain notifier

[ellemouton]

Summary

The ChainNotifier confirmation/spend gRPC streams drop two pieces of
information that lnd already tracks internally:

1. Re-org depth — ConfirmationEvent.NegativeConf carries the depth as an
   int32, but the server discarded it and sent an empty Reorg{} (the proto
   message was a // TODO(wilmer) placeholder).
2. Finality / Done — ConfirmationEvent.Done / SpendEvent.Done fire once
   a watch is past re-org safety depth, but the server conveyed this only by
   returning (closing the stream), which a client cannot distinguish from an
   unrelated teardown. There was no Done event in the ConfEvent/SpendEvent
   oneof at all.

This makes the chain-event surface reversible and terminable end to end, which
reorg-safety-aware consumers (e.g. anything that gates terminal state eviction
on reaching re-org safety depth) need.

Changes

• proto: add a depth field to Reorg, add a new Done message, and add
  done to both the ConfEvent and SpendEvent oneofs. Additive and
  backwards compatible.
• chain_server.go: forward the real re-org depth on the confirmation path's
  Reorg, and send an explicit Done event before exiting on both the
  confirmation and spend paths. Spend-side re-org depth is left at 0 because the
  notifier does not track one internally.

Notes

• Spend-side re-org depth is intentionally left at 0: the notifier's
  SpendEvent.Reorg is a bare signal with no depth, so surfacing it would
  require a deeper notifier change.

[github-actions]

🟠 PR Severity: HIGH

gh pr files | 5 files | 288 lines changed (92 excluding generated/docs)

🟠 High (3 files)

• lnrpc/chainrpc/chain_server.go - lnrpc/* RPC/API definition logic (chain notifier server implementation)
• lnrpc/chainrpc/chainnotifier.proto - lnrpc/* wire/API definition change (new proto fields)
• lnrpc/chainrpc/chainnotifier.swagger.json - lnrpc/* generated API surface (REST gateway spec)

🟢 Low (2 files)

• docs/release-notes/release-notes-0.21.0.md - release notes only
• lnrpc/chainrpc/chainnotifier.pb.go - auto-generated protobuf code (excluded from line/file counting)

Analysis

The highest-severity files touched are under lnrpc/chainrpc/*, which falls under the lnrpc/* category (RPC/API definitions) — classified as HIGH severity. This includes a .proto change (new/changed RPC fields) and the corresponding server-side handler in chain_server.go, plus the generated swagger spec.

No bump conditions apply: only 4 non-generated files changed (well under the 20-file threshold), ~92 non-generated/non-doc lines changed (well under the 500-line threshold), and no critical packages (lnwallet, htlcswitch, contractcourt, sweep, etc.) are touched.

Since this PR modifies a public RPC API surface (chainnotifier), it warrants review from someone familiar with the lnrpc API conventions and backward-compatibility concerns, consistent with HIGH severity.

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