CTX-PROOF 2/2: parity fixture proving rem lengths in the music player's flex and positioned contexts

[thejackshelton]

CTX-PROOF 2/2 (music player). The north-star check refuses 19 declarations per native target with DRAGON_UNPROVEN_CONTEXT. Sixteen of them are rem lengths in contexts where px (or calc) was proven and rem had no fixture. This PR adds one parity fixture that uses those exact values in those contexts. No engine, compiler, gate or tolerance change. Profile rows come from regen's profile-rows step.

What changed

• New fixture group rem-contexts (packages/parity/src/fixture-groups/rem-contexts.ts; named so its GROUPS line is not next to CTX-PROOF 1/2: parity fixtures proving auto margins and auto insets in block, flex and positioned contexts #179's ctx-proof line, one line in GROUPS) with ctx-proof-rem, run in ltr and rtl:
  • padding: 0.5rem (and a four-value rem padding) plus min-height: 2.5rem on position: relative flex-row items, as on .library-button.
  • So these values change a frame, row r3 uses align-items: flex-start, so each item's height comes from its content:
    • b3's content is 30px, so min-height: 2.5rem sets its height to 40px.
    • b4 has no min-height; padding-bottom: 1.5rem makes it 42px.
    • b5 is 54px: padding-bottom: 1rem under a 30px child.
  • These come from Chrome in both directions, and the engine lane matches them. The stretched row r1 alone could not catch an ignored min-height or a wrong rem padding-bottom (review of 061e8d0).
  • padding: 1rem (and a four-value rem padding) on absolute boxes in a block containing block, as on .animate-track.
  • width: 18rem, max-width: 18rem / 12.5rem on flex-column items, as on .time-control and .song-title.
  • left: 1rem, top: 0.75rem on a static flex-column item. That is the context .mini-video-shell reaches once its position: fixed is blanked. The fixture proves the static case (insets have no effect, Chrome-equal). The fixed box itself stays refused under its own diagnostic until POSX-f.
  • min-width: 2.5rem / 6rem on flex-row items, as on .time-control p.

Not covered (stay refused, for a reason other than a missing fixture)

• overflow-x: hidden on html: viewport propagation (OVFL).
• display: inline-flex / inline-block in block flow: atomic inlines (INL2a, INL2a 1/2: atomic inlines in the layout engine (inline-block, inline-flex) #112/INL2a 2/2: inline-block and inline-flex in the compiler, with Chrome fixtures #115).
• font-family pinned in a text-in-flex-item context (web only): real fonts (TXT1a).

Verification

• A one-off runner (not committed) ran the fixture through runFixture with live Chrome (liveAuthored, profiles derive). Both cases pass linux-dragon-layout and chrome-dual. Their ios keys include all 16 refused keys, in both directions.
• registry-claims.test.ts: pass. pnpm typecheck: pass.
• Outputs come from the regen label on CI, in their own commit.

Result (on the tree merged with master)

• Profile rows: +28 per target (ios, android, web: <length-rem> for padding-*, min-height, min-width, width, max-width, left and top in the contexts above, ltr and rtl). Rows removed: 0. Status changes: 0.
• north-star-check.json: DRAGON_UNPROVEN_CONTEXT goes from 19 to 3 per native target (web from 21 to 5). Errors go from 286 to 238. Declarations supported on both native targets go from 193 to 196 of 289 applicable. The other refused declarations still carry a different error, such as position: fixed or transitions.
• Coverage-rank headline: unchanged at 37.4% native. None of its probes uses rem in these contexts.
• After regen: registry-claims, north-star-accounting, north-star-tree and profile-index tests: 18/18 pass. pnpm typecheck: pass.

Fixtures added

This adds 1 new fixture. No fixture, test, check or tolerance was changed or removed.

🤖 Generated with Claude Code

[thejackshelton]

Landing stopped at step test (pnpm land)

packages/parity/test/hit-capture-identity.test.ts, rerun alone on a quiet machine, exited 1 (log /tmp/land-180-test-solo-1.log)
 ❯ packages/parity/test/hit-capture-identity.test.ts:64:56
     62|   it('holds only SELD-R1b fixtures\' files beyond the base', () => {
     63|     const extra = IDENTITY_ROOTS.flatMap(walk).filter((p) => (p.endsWi…
     64|     expect(extra.filter((p) => !IDENTITY_NEW.test(p))).toEqual([]);
       |                                                        ^
     65|     expect(extra.length).toBeGreaterThan(0);
     66|   });

⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯[1/1]⎯


 Test Files  1 failed (1)
      Tests  1 failed | 5 passed (6)
   Start at  14:31:45
   Duration  2.64s (transform 1.54s, setup 0ms, import 2.07s, tests 502ms, environment 0ms)

Found by bisecting the batch #188 #180 #176 #135 (3 proofs): master with #188 passes, adding #180 fails.

Fix the cause, then hand #180 back to the landing queue with the new clean head.