facet_chart: add opt-in linked panel interactions

[FarhanAliRaza]

Motivation

• Facet panels should be able to synchronize navigation and brushing so users can compare the same data window or selection across panels while preserving the current default of independent panel interactions.

Description

• Add link: Optional[str] = None and link_select: bool = False to facet_chart/FacetChart with eager validation of allowed values (None, "x", "y", "both").
• Make linked axes share their initial domain at build time and set a browser-local link_group + link_axes on per-panel Figure instances when linking is requested, preserving existing share_x/share_y behavior for initial-domain alignment.
• Extend the client to broadcast and receive linked selection predicates (range, polygon, and clear) over the existing BroadcastChannel, add _broadcastLinkedSelection, and update selection handlers to accept opts that suppress duplicate dispatch/broadcast when applying incoming linked selections.
• Surface the new options in docs (docs/guides/dashboards-and-linked-views.md) and add/adjust tests covering independent defaults, explicit link choices, link-implied domain sharing, invalid link values, and selection linking logic.

Testing

• Built client bundle with node js/build.mjs and ran targeted suites PYTHONPATH=python pytest -q tests/test_facets.py tests/test_components.py tests/test_static_client_security.py, which all passed (138 tests).
• Ran the full test suite with PYTHONPATH=python pytest -q, which succeeded (1,835 passed, 77 skipped).
• Verified static client contract tests and fixed a transient client-regression discovered during the first run before the final verification, after which the security/contract tests passed.

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Closes #92

[codspeed-hq]

Merging this PR will not alter performance

✅ 94 untouched benchmarks
⏩ 1 skipped benchmark¹

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_{Comparing codex/add-linked-zoom-and-selection-for-facet_chart (6a75f8d) with main (e3311dd)}

Footnotes

1. 1 benchmark was skipped, so the baseline result was used instead. If it was deleted from the codebase, click here and archive it to remove it from the performance reports. ↩

[Alek99]

Review findings:

• [P1] python/xy/components.py:3306: link=None removes the runtime linking that existing share_x=True/share_y=True facets receive today. This is a breaking change against the prior tests/docs; preserve it or provide an explicit migration/versioning path.
• [P1] js/src/50_chartview.js:442: incoming linked selections use _selectLocal*, which skips density traces and cannot select drilled GPU data. Receiver facets do not highlight, and selection callbacks only see the source facet rather than the requested cross-panel result.
• [P2] js/src/50_chartview.js:458: selection-only links still broadcast/apply view events with an empty link_axes, producing redundant LOD requests and callbacks on untouched facets. Avoid broadcasting view updates when no axes are linked.