refactor(core): one owner for interaction sessions, editors and context menus

[chh-ay]

Summary

Architecture layer: one private owner for pointer, editor and context-menu sessions.

The grid spread pointer, editor and context-menu ownership across GridImpl and a loose input controller, so gestures, editors and context menus could outlive the state they were captured against. A single InteractionSessionController now owns all three. The review's five defects are fixed as one change, and the hot paths the review touched were measured:

• Resize previews stay out of the document: row and column resize paint a preview while dragging and commit one undoable change on release. Before, each preview step mutated the document, so a cancelled drag left changes behind.
• Gestures end with their context: an active pointer gesture is cancelled on sheet, view, structure or read-only changes, and a second pointer cannot take over a drag in progress.
• Editors keep their cell: stock and custom editors both commit to the document cell they opened on, through sorting and filtering, instead of resolving the target differently.
• Rejected fills leave the selection alone: a fill whose admission is rejected no longer moves the selection to cells it did not write.
• Selection before edit-begin: selection changes are reported before the edit-begin event, and reentrant handlers no longer reopen a stale editor.
• No public change: the same events in the same order with the same payloads. Behavior that the review classified as a defect changed; published signatures did not.
• Edit notifications: the bridge builds an edit notification only while at least one consumer is subscribed, so applications without an edit handler stop paying for the address list.

Interaction	Baseline	Branch	Change	A/A band
Edit commit	0.202 ms	0.194 ms	−3.7%	2.4%
Column resize	0.168 ms	0.151 ms	−10.4%	2.6%
Row resize	0.218 ms	0.144 ms	−34.2%	3.3%
Selection	0.0077 ms	0.0078 ms	+0.5%	2.1%
Get cell	0.0402 ms	0.0404 ms	+0.5%	3.3%
Lookup median (ns)	20.545	20.493	−0.3%	1.7%
Lookup p95 (ns)	235.2	241.2	+2.5%	9.3%
Inverse index bytes	4,000,000	4,000,000	0%	0%
Dirty-100 bytes	5,679	5,679	0%	0%
Owned cold long task	0 ms	0 ms	0%	0%
Unattributed cold long task	89.09 ms	88.43 ms	−0.7%	1.8%
Usable time	1,152.2 ms	1,150.8 ms	−0.1%	14.9%

Gate: baseline 4997890, five A/A control rounds and five alternating rounds, each scenario in a fresh Bun process pinned to CPU 4 under the shared lock. Every changed path is at or better than its A/A band and no untouched workload is slower. Owned cold long task stays at zero, under the 50 ms ceiling; the inverse index and dirty-100 bytes stay at their ceilings.

The layer also adds the interaction benchmark producer (bench/src/interaction-sessions-bench.ts) and documents its paired-noise protocol, so the release capture regenerates the interaction-current-v2 evidence from the checked-in producer.

Changesets

interaction-sessions: @sheetwrite/core patch.

Verification

• Core and docs tests: 601 across 75 files; typecheck, lint (two existing unrelated warnings), api:check and docs:check pass.
• Full Chromium suite: 83 tests pass on this branch, including the new interaction and example specs.
• New tests: input.test.ts covers cancellation and resize-history contracts; semantic-grid.test.ts covers the editor-target and selection contracts; examples.spec.ts drives the browser paths.

[chh-ay]

Landed on develop in the 0.5.0 release squash: #112 (55cfa82).