fix(accessibility): preserve forced-colors state cues

[seonghobae]

Purpose

Implement issue #150 as a bounded forced-colors/high-contrast accessibility slice without changing Inkspan's normal light/dark theme behavior or host authority boundaries.

Exact test-first lineage

• Protected base at branch creation and current live protected tip: main@50ac98cfa0ad9e8dd75f93ca437a5679fed4d804.
• Initial RED head: d89bd41dabca47a00d486e847eed0d3df8b5456b. Exact CI run 31412302233, build-and-test job 93533014788, checked out that head and proved the missing forced-colors system-state mappings.
• Initial implementation head 23a24f5f20475543e51c97a83a5dc7fc08db9ddb made the requested system-color/browser contract green, but a later audit found the authored @media (forced-colors: active) overrides were split before later equal-specificity base rules, leaving a real CSS-cascade regression risk.
• Cascade RED head bcd993a1d521d3f44fe07eaeaa5acda10dce3d6b changed only src/forcedColorsStyles.test.ts. Exact CI run 31416835830, build-and-test job 93547761779, checked out that head and failed the intended boundary because two screen forced-colors blocks existed.
• Source correction head bd109253141bcda7b0514de95d25f43ee2710602 consolidated the screen forced-colors overrides after the normal screen state rules. Its browser/Office/security/SAST jobs passed, while CI run 31417191422 exposed a test-scoping defect: the order assertion counted later @media print selectors that cannot override screen forced-colors behavior.
• Test-scope head 940c2e1341ad46c66f1cdc4cfab3603fd7dfc1aa correctly excluded print media but still counted selectors inside the forced-colors block itself. Exact CI run 31417569588, build-and-test job 93550088295, failed only that over-broad assertion while the other 825 tests passed.
• Current exact GREEN head: 862db1d4ec2f910fcc65dbd3aabdfcd49877d98d. The final contract identifies the balanced forced-colors media block, requires relevant base screen selectors before it, and rejects matching normal screen rules after it without confusing forced-colors or print declarations.

Implemented boundary

Inside one final screen @media (forced-colors: active) layer, Inkspan uses CSS system colors to preserve editor/container boundaries, toolbar and collaboration status chrome, toolbar grouping, keyboard focus, active and disabled control states, authored document links, blockquote/placeholder cues, code/pre/table borders, and collaboration cursors. Disabled buttons no longer rely on opacity as their forced-colors state cue. Normal light/dark theme declarations remain unchanged outside that media query, and Inkspan does not opt out of user-agent forced-color adjustment.

The deterministic stylesheet contract now also guards cascade placement. tests/browser/specs/forced-colors.browser.spec.ts supplies network-blocked real-engine evidence via Playwright forced-colors emulation for media activation, keyboard focus, disabled-state opacity, structural borders, underlined links, collaboration cursor/label visibility, and the editable document surface's own focus-visible outline.

The slice adds no persistence, transport, credentials, model use, authorization, tenant, network, provider, room, or durable-audit authority.

Exact-head evidence

For unchanged exact head 862db1d4ec2f910fcc65dbd3aabdfcd49877d98d:

• CI run 31417814093: success;
• Security Scan run 31417815005: success;
• SAST Semgrep run 31417814222: success;
• CI build-and-test passed typecheck, all 826 tests with exact 100% owned-production coverage, library build, packed-package consumer verification, and demo build;
• the cross-engine Playwright 1.62.0 job passed, including the network-blocked forced-colors browser contract;
• Office Python 3.11, 3.12, 3.13 and 3.14 test/100% branch-coverage/docstring/wheel/package jobs all passed;
• CodeRabbit status is successful on this exact head; and
• fresh review inspection shows no submitted formal reviews and no unresolved inline review threads. Automated status is not represented as independent formal approval.

Release dependency

Issue #118 still owns the exact 0.6.0 stable-candidate operational boundary. Keep this PR Draft and unmerged until that release boundary closes; moving protected main first would invalidate the current stable-candidate identity required by the protected release workflow. After #118 closes, refetch the then-current protected tip, reacquire live-base ancestry and exact-head gates, reconcile canonical next-release documentation/traceability, and only then evaluate Ready/merge status.

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