Fix Select Base UI parity regressions

[Ernxst]

Harness: Codex
Agent: none
Model: gpt-6-astra

Task statements

1. Fix the Select behaviour differences confirmed by Base UI parity reproductions, with regression tests and updated reference documentation. (Working-directory and workflow instructions omitted.)

2. Open this first PR for the six confirmed fixes while the complete Base UI Select inventory continues in a follow-up PR. (Working-directory and workflow instructions omitted.)

This change brings Select group semantics, valueless item highlighting, keyboard focus and opening navigation, ItemText context errors, and selected-item removal behaviour in line with Base UI 1.8.0.

The wider Base UI Select inventory is still in progress. A follow-up PR will account for all 288 source test rows, report the final counts, and update tracking issue #841.

Verification

• On origin/main, the selected regression run reported 6 failed, 2 passed, 84 skipped.
• On this branch, select-parity.test.tsx passes all 93 tests, including the six repaired behaviours.
• The Select, Combobox, Tooltip and Dialog floating-layer suites passed sequentially: 93, 1, 17, 21 and 4 tests respectively.
• React typecheck passed.
• No screenshot goldens changed. Screenshot capture was unavailable because this BB environment has no active display.

Closes #872
Closes #895
Closes #896
Closes #897
Closes #898
Closes #900