Add cache-invalidation tests for dynamic containing-block changes

[nicoburns]

Objective

Add hand-written test coverage verifying that out-of-flow hoisting stays correct across relayouts with hot caches when the containing-block structure changes dynamically. Test-only change (no RELEASES.md update needed).

New tests in tests/hand_written/oof_hoisting.rs:

• claim_change_relayouts_with_hot_cache: an intermediate ancestor toggles position: static ↔ relative; the abspos box must be claimed by (and positioned relative to) the new containing block, and move back when the style is reverted.
• fixed_claim_change_relayouts_with_hot_cache: a fixed box inside an abspos ancestor is claimed by the root; dirtying only the fixed box still relays it out via the root.
• content_change_inside_hoisted_subtree: growing a leaf inside a hoisted abspos subtree resizes the hoisted box even when siblings hit the cache.

Context

The CSS WPT tests for dynamic containing-block changes (e.g. transform-containing-block-dynamic-1b) are script-driven and can't run in the current test infrastructure, so these scenarios are ported to hand-written Rust tests instead. Companion Blitz-side tests (style-attribute mutation, :hover-driven restyles, DOM insertion/removal) are added on DioxusLabs/blitz#763.

Stacked on #1144. All tests pass against the current implementation — this locks in the cache-invalidation behavior rather than fixing a bug.

Link to Devin session: https://dioxus.staging.devinenterprise.com/sessions/65c5463061c8403bade7888271d5eeb4
Open in Devin Desktop: https://dioxus.staging.devinenterprise.com/desktop/session/65c5463061c8403bade7888271d5eeb4?variant=devin-insiders
Requested by: @nicoburns

[staging-devin-ai-integration]

🤖 Devin AI Engineer

I'll be helping with this pull request! Here's what you should know:

✅ I will automatically:

• Address comments on this PR that start with 'DevinAI' or '@devin'.
• Look at CI failures and help fix them

Note: I can only respond to comments from users who have write access to this repository.

⚙️ Control Options:

• Disable automatic comment, CI, and merge conflict monitoring

[staging-devin-ai-integration]

Absorbed into #1144 (the PerformLayout-only gating fix and the cache-invalidation tests now live there).