Abspos: apply safe static alignment within the IMCB

[nicoburns]

Objective

Match Chrome/WPT for safe static alignment when the containing block is outside the originating grid or flex container.

Context

With both insets auto, check margin-box overflow against the existing inset-modified containing block (IMCB), not the static-position rectangle. On overflow, align to the containing block's logical start within the IMCB; otherwise retain the original static alignment. Sizing, margins, non-auto inset paths, default/unsafe alignment, and the standalone resolve_static_offset API are unchanged. Nine net production lines; no new allocation or per-node storage.

This intentionally chooses Chrome's interpretation of CSSWG #11934, which remains unresolved; Firefox and Safari fail the two tentative grid tests too. Independent of the collapsed-gutter fix in #1266.

Validation:

• Chrome-generated grid, flexbox, and block fixtures only: margins, fitting/overflowing boxes, RTL, row-reverse, same/outer containing blocks, and unchanged unsafe/default alignment. Fixture names include tentative and comments link their upstream WPTs or related tentative coverage. All 12 generated variants fail before and pass after.
• cargo test --all-features: 185 unit, 146 handwritten, 6377 generated, and 5 doctests passed (4 existing ignored). No new non-generated tests.
• Formatting, workspace Clippy, and no-std grid/flex/block check passed.
• Full Blitz CSS WPT A/B: 3 newly passing tests, no pass-to-fail regressions. Grid safe-outer-cb-001 and 003, and flex safe-outer-cb-003, now pass.
• Combined with Grid: resolve absolute edges across collapsed auto-fit gutters #1266: all 3663 grid-abspos subtests pass; full CSS WPT has no pass-to-fail regressions.

Link to Devin session: https://dioxus.staging.devinenterprise.com/sessions/87b105dd2f0549cf9edddc0f14d12ea0
Open in Devin Desktop: https://dioxus.staging.devinenterprise.com/desktop/session/87b105dd2f0549cf9edddc0f14d12ea0?variant=devin-insiders
Requested by: @nicoburns

[staging-devin-ai-integration]

I'll fix CI failures and address comments from users with write access that start with 'Devin'.

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