#23489 composer-draft-wrap test: share a page pool so the file fits the 120 s limit

Fixes the webviews shard timeout in test/composer-draft-wrap.test.ts ("file exceeded its 120 s limit"). This is the same cause as composer-type-scale (#23450), not a flake.

Cause: each of the 32 cases opened its own page and context, and a fresh page refetches about 700 gallery modules. Run one after another after the Vite warm-up, the file took longer than CMUX_WEB_TEST_FILE_TIMEOUT (120 s).

Fix: the same as #23450.

• The cases run through a 4-page FIFO pool started in beforeAll. Each test awaits its own result, and the assertions are unchanged.
• The two fixed 50 ms sleeps now wait for a painted frame (installPaintedFrames / cmuxPaintFrame). That drops the file's 4-wait row from fixed-waits-baseline.tsv.

Timings (Linux, Chromium plus WebKit, wall time including Vite and browser startup):

	wall	result
base, alone	149 s, 153 s (instrumented: warm-up done at 39.8 s, then 32 tests at 2.6–5.7 s each, 156.7 s total)	32/32, over the 120 s limit
this PR, alone	82 s, 76 s, 76 s	32/32
this PR, 16 CPU spinners (load 84 on 32 cores)	130 s	32/32

bunx vp check and check:fixed-waits pass locally. check:browser-lanes fails on base at diff-paint-perf.test.ts:107, a file this PR doesn't touch.

Agent session: huge-blue

🤖 Generated with Claude Code

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Summary by cubic

Fixes the composer-draft-wrap.test.ts timeout (file exceeded its 120 s limit) by running the 32 cases through a shared 4-page pool instead of opening a fresh page per test.

Bug Fixes

• A fresh page refetches ~700 gallery modules, so 32 sequential pages after Vite's warm-up took 149–157 s; the pool runs them in parallel with assertions unchanged.
• The two fixed 50 ms sleeps now wait for a painted frame, which also drops the file's row from fixed-waits-baseline.tsv.
• Wall time drops to about 76–82 s alone and 130 s under CPU load, with all 32 cases passing.