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fix(screenshot): never let the text walk fail a capture (#845)

* fix(screenshot): never let the text walk fail a capture

`waitForReady`'s snapshot built a TreeWalker over `document.body ||
document.documentElement`. A document that is mid-parse or detached has
neither, and `createTreeWalker` throws on a null root. Because that is not a
context-destroyed error, the gate rethrows it by design, so a survivable DOM
state failed the whole render.

Observed on production 3.42.8, three times in six hours, on documents of 6, 25
and 426 pages:

  chunk:failed ... message="Failed to execute 'createTreeWalker' on 'Document'"

Guard the root, and wrap the walk so a DOM mutating underneath it keeps the
count taken so far. `text` is only a paint signal: not being able to read it
means "no text seen yet", which leaves the gate conservative — it re-polls, or
falls through to the blank-page screenshot check — instead of failing a capture
that would otherwise succeed.

The rethrow for genuine evaluation errors is unchanged; its test still passes.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

* fix(screenshot): guard every root dereference in the snapshot

The #845 guard covered the TreeWalker root but the same null-documentElement
state still threw one line before the return: `scrollHeight` (and `clientWidth`/
`clientHeight` behind a zero innerWidth) dereferenced the root unguarded, so the
production error moved instead of dying. Resolve the root once and degrade every
read to zero; a transient height 0 just resets the quiet window and re-polls.

Live regression test reproduces the exact state (document.removeChild of the
root) and fails without the guard.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01P1ecHpTgQwD4cH7vutmCon

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Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

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