Parse legacy CC-Things-Tag-* and recurring instance IDs

[BenEmdon]

Fixes #10.

Accounts with data from old Things versions cannot sync at all. Two legacy ID formats break ThingsId::from_str, and a single bad entry aborts the whole append-log fold:

• CC-Things-Tag-Office (and Errand, Home, Important, Pending): built-in tag IDs, appear in tg fields.
• <UUID>-<YYYYMMDD>: materialized recurring-task instance IDs, template UUID plus instance date. Appear as object keys and in tombstone dloid fields.

Repro: run things3 today (any command that syncs) against such an account. It fails with e.g. Corrupt log entry at .../things.log byte 42872: invalid Things ID: "CC-Things-Tag-Office".

The fix canonicalizes both formats the same way ACTIONGROUP- IDs already are: digest the full string with SHA1 truncated to 16 bytes, so entity keys and references agree deterministically. For the instance form it only applies when the base parses as a UUID and the suffix is exactly 8 digits, so genuinely malformed IDs still error. Instances stay distinct from each other and from their template (the template link travels in the rt field as a bare UUID).

Tests: 6 new unit tests covering determinism, distinctness, serde roundtrip, and rejection of bad inputs. Full suite passes (cargo test --locked).

One caveat: unlike ACTIONGROUP-, there is no bare-UUID form the server recognizes for these IDs, so writes would re-encode the digested form. Reads are unaffected.

[evanpurkhiser]

nit and maybe clippy will catch, but I think you could write this as

Suggested change

	if let Some((base, date)) = s.rsplit_once('-') {
	if date.len() == 8
	&& date.bytes().all(|b| b.is_ascii_digit())
	&& Uuid::parse_str(base).is_ok()
	{
	let digest = Sha1::digest(s.as_bytes());
	let mut bytes = [0u8; 16];
	bytes.copy_from_slice(&digest[..16]);
	return Ok(ThingsId(bytes));
	}
	}
	if let Some((base, date)) = s.rsplit_once('-')
	&& date.len() == 8
	&& date.bytes().all(|b| b.is_ascii_digit())
	&& Uuid::parse_str(base).is_ok()
	{
	let digest = Sha1::digest(s.as_bytes());
	let mut bytes = [0u8; 16];
	bytes.copy_from_slice(&digest[..16]);
	return Ok(ThingsId(bytes));
	}

via let chains https://blog.rust-lang.org/2025/06/26/Rust-1.88.0/#let-chains

[evanpurkhiser]

Thanks very much for this fix!

If you can address the lint errors we can get this merged!