Normalize YAML timestamp values for gojq - #77

Description

Closes #76. YAML timestamp values currently remain time.Time, which gojq cannot encode and can panic on, even though timestamp keys already have a stable string representation. Normalize values with the same existing timestampString helper so top-level, nested, sequence and multi-document values are safe for gojq and agree with keys.

Changes

• Fixed the timestamp-value normalization bug without changing the existing timestamp-key format.
• Added regressions for dates, UTC, offsets/fractions, recursive shapes, multi-document inputs, gojq encoding and key/value parity; quoted strings and null remain controls.

Testing

On macOS with Go 1.25.12:

• go test -count=1 ./..., go vet ./..., and build passed.
• make test passed, including the shell integration harness (347 passed, 171 skipped by that harness).
• go test -race -count=1 ./codec/... passed.
• Built CLI: printf 'a: 2024-01-01\n' | qq -iyaml tojson completes with the date string instead of panicking.

Additional Notes

Full go test -race -count=1 ./... fails in the pre-existing CLI tests' concurrent stdout/Version handling. Running go test -race -count=1 ./cli in a separate, unmodified worktree at base 0ca9d3c3a9a1206259c39b99e927df81a29c22e2 reproduces that race. It is not reported as passing or changed here. Linux/Windows were not run locally. Timestamp lexical spellings are normalized using the existing formatter, not preserved verbatim.