fix(context-menu): restore pane header targeting

[danshapiro]

Summary

• restore pane-specific context menus for right-click and primary-button context gestures on pane headers
• preserve data-context="pane-header", add exact tab/pane identity, and resolve pane headers through the existing pane target
• add direct parser and rendered-attribute coverage, including safe handling of missing identity fields

Root cause

The pane-header marker stopped context lookup at the header, but the parser did not recognize that marker and the header carried no tab or pane IDs. Parsing therefore returned no pane target and the provider fell back to the global application menu.

Test plan

• Reproduced on main: pane-context-menu-stability reported 2 failed and 4 passed
• Focused fixed behavior: 84 passed
• Related and new unit coverage: 149 passed
• npm run typecheck: passed
• npm run lint: passed with 0 errors (11 unrelated existing warnings)
• npm run test:e2e:a11y-gate:deny: passed
• Fresh npm run check: passed
  • client: 5,019 passed; 8 skipped
  • server: 4,924 passed; 29 skipped
  • electron: 350 passed
• Earlier broad-run deadline flakes were rechecked: all three tests passed together and in three repeated focused runs before the fresh full pass

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