fix(chain): restore first seen timestamps in TxGraph

[NaitikVerma6776]

Fixes #2270.

TxGraph::apply_changeset was restoring last_seen but not first_seen. Replaying last_seen through insert_seen_at could therefore incorrectly set first_seen to the last_seen timestamp.

This change restores first_seen and last_seen independently when applying a ChangeSet.

A regression test was added to verify that distinct first_seen and last_seen timestamps survive applying and reloading a changeset.

Tests

• cargo +nightly fmt --all -- --check — passed
• git diff --check — passed
• Full Rust test suite could not be run locally because the Windows environment does not have the MSVC link.exe linker installed.

Contribution checklist

• Regression test added
• Changes are limited to the relevant chain implementation and test
• Commit is GPG signed

[evanlinjin]

Please also sign your commits.

[evanlinjin]

Suggested change

	// Restore first_seen and last_seen independently. Replaying last_seen via
	// `insert_seen_at` on an empty graph would set first_seen to last_seen.
	for (txid, seen_at) in changeset.first_seen {
	let _ = self.update_first_seen(txid, seen_at);
	}
	for (txid, seen_at) in changeset.last_seen {
	let _ = self.insert_seen_at(txid, seen_at);
	let _ = self.update_last_seen(txid, seen_at);
	}
	// Restore first_seen and last_seen independently. Replaying last_seen via
	// `insert_seen_at` on an empty graph would set first_seen to last_seen.
	for (txid, seen_at) in changeset.first_seen {
	let _ = self.insert_seen_at(txid, seen_at);
	}
	for (txid, seen_at) in changeset.last_seen {
	let _ = self.insert_seen_at(txid, seen_at);
	}

It's safer to call insert_seen_at for both changeset fields to avoid the last_seen >= first_seen contract from being violated.

[NaitikVerma6776]

@evanlinjin Please review the changes
happy to receive feedback