fix: resolve RFC 9557 compliance findings from expert review

[8beeeaaat]

Summary

Fixes all findings from the v0.4.0 pre-release RFC 9557 compliance review (blocking PR #25):

Finding	Fix
Major-1: Format emitted offset zones as [+0900] (RFC grammar violation, round-trip loss)	Keep +09:00 as the FixedZone name (RFC 9557 §1.2 serialization); formatOffsetName removed
Major-2: duplicate critical suffix keys not rejected (§3.3 MUST)	Duplicate key errors with ErrCriticalExtension when either occurrence is critical
Major-3: unknown critical keys accepted even in strict mode	Strict mode rejects critical keys other than u-ca; non-strict delegation via Critical map documented in README
Minor-4: second time-zone annotation slipped through after an ignored unknown zone	Parse state (seenTimezone) decoupled from ext.Location
Minor-5: suffix = [time-zone] *suffix-tag ordering unenforced	Time-zone annotation after a suffix tag → ErrInvalidSuffix in both modes
Minor-6: Format rejected CriticalLocation even when the timestamp's own zone would be emitted	Critical flag now applies to the fallback zone; error only when no zone is available
Code quality: ParseError lacked Unwrap	Added; sentinel errors now match with errors.Is

Spec-conformance tests added

• RFC §3.3 example strings: [!u-ca=chinese][u-ca=japanese], [u-ca=chinese][!u-ca=japanese], [!knort=blargel]
• RFC §1.2 offset example: 2022-07-08T00:14:07+08:45[+08:45]
• Parse → Format → Parse round trips for offset annotations incl. critical variant
• Grammar-ordering and ignored-zone duplicate cases
• errors.Is matching through ParseError.Unwrap

Verification

• go test -race ./... passes (all packages)
• golangci-lint run: 0 issues
• go vet, gofmt: clean

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 95.45455% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 94.45%. Comparing base (56017bb) to head (ebd7fe8).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
ixdtf.go	95.45%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #26      +/-   ##
==========================================
- Coverage   94.76%   94.45%   -0.31%     
==========================================
  Files           1        1              
  Lines         382      397      +15     
==========================================
+ Hits          362      375      +13     
- Misses         12       13       +1     
- Partials        8        9       +1     

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[8beeeaaat]

@codex review

[Copilot]

Pull request overview

This PR addresses RFC 9557 compliance findings for IXDTF parsing/formatting, including stricter grammar enforcement for suffix ordering/duplication rules, correct offset time-zone serialization, and improved error interoperability via ParseError.Unwrap.

Changes:

• Fix offset time-zone annotation round-tripping by preserving RFC 3339 +HH:MM form as the FixedZone name and updating offset-name detection accordingly.
• Enforce RFC 9557 §4.1 suffix grammar (single time-zone annotation, must precede tags) and RFC 9557 §3.3 handling for critical duplicate/unknown keys.
• Add spec-conformance tests (round trips, ordering violations, errors.Is via ParseError.Unwrap) and document strict vs non-strict critical-tag responsibilities in the README.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

File	Description
README.md	Documents strict/non-strict responsibility for unknown critical tags and clarifies offset-annotation behavior and suffix ordering constraints.
ixdtf.go	Implements RFC 9557 compliance fixes: offset zone naming, stricter suffix parsing state, critical-key duplicate/unknown handling, and ParseError.Unwrap.
ixdtf_test.go	Adds/updates tests for offset annotation round-trips, suffix ordering errors, critical duplicates/unknown critical keys, and errors.Is behavior.
ixdtf_internal_test.go	Removes tests tied to the deleted formatOffsetName and updates offset-name detection expectations and parser helper callsites.

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