test: isolate NO_COLOR palette equality from session age

[SUaDtL]

Summary

Keep the NO_COLOR palette equality test independent of mutable user-ledger state. The theme-only payload now omits its unused session id, so every renderer subprocess produces a call-local age 0s instead of racing a live age boundary.

Shipped statusline behavior is unchanged.

Closes #355

Verification

• Deterministic red proof reproduced age 59s versus age 1m as the sole byte difference.
• Target test passed 30 of 30 fresh-process repetitions on Linux.
• Full hook suite passed 967 tests with 1 intentional skip on Linux.
• All 25 .github/scripts/test_*.py suites passed.
• Python compile, whitespace validation, and staged secrets scan passed.
• Independent coverage review passed with no findings.

Tradeoff

The comparison no longer exercises session-dependent ledger output. Under the project conflict hierarchy, that behavior is outside this palette test's obligation and remains covered by dedicated ledger and statusline tests; keeping it here made a theme assertion depend on wall-clock timing.

[SUaDtL]

Superseded by #313, the single hackathon review surface. This PR's recorded head and changes are represented in #313's exact 16-PR / 29-commit / 126-path source manifest. PR #313 passed final exact-head CI at commit 6173b1d and remains open and unmerged. Closing this source PR to avoid parallel review; please continue review and discussion on #313.