chore(bundle-size): add fixtures for Focusable find* methods

[layershifter]

Summary

Adds four bundle-size fixtures used by the monosize harness — one per Focusable find variant, so each entry point can be measured independently:

• focusableFindAll.fixture.js
• focusableFindLast.fixture.js
• focusableFindNext.fixture.js
• focusableFindPrev.fixture.js

Purely additive — only files under bundle-size/ are added, no shipped code changes.

The matching fixture for getModalizer with controlTab: false is deferred to the later stack PR that introduces getRootDummyInputs, since the fixture needs that export to exist.

Stack context

First PR in the bundle-size stack. Parent: master.

[github-actions]

📊 Bundle size report

Package & Exports	Baseline (minified/GZIP)	PR	Change
tabster focusable.findAll	0 B 0 B	39.784 kB 11.9 kB	🆕 New entry
tabster focusable.findLast	0 B 0 B	39.785 kB 11.9 kB	🆕 New entry
tabster focusable.findNext	0 B 0 B	39.785 kB 11.9 kB	🆕 New entry
tabster focusable.findPrev	0 B 0 B	39.785 kB 11.9 kB	🆕 New entry

Unchanged fixtures

Package & Exports	Size (minified/GZIP)
tabster all exports	113.84 kB 30.937 kB
tabster createTabster (core)	39.761 kB 11.892 kB
tabster getCrossOrigin	110.623 kB 30.244 kB
tabster getDeloser	49.268 kB 14.234 kB
tabster getGroupper	47 kB 13.593 kB
tabster getModalizer	49.072 kB 14.378 kB
tabster getMover	54.653 kB 15.887 kB
tabster getObservedElement	45.564 kB 13.484 kB
tabster getOutline	48.867 kB 14.216 kB
tabster getRestorer	42.518 kB 12.52 kB

_{🤖 This report was generated against a579ebbd50e37f1565551549fe57bbc9ddafab64}

[layershifter]

Stack overview

This PR is the first in a 14-PR stack splitting up the bundle-size refactor work (replaces #531). Land in order from top to bottom; each PR's diff against master is cumulative on top of every prior PR in the chain. Reviewers can subtract the parent's diff from the current PR's diff to see what's new in each one — each PR description has a "Stack context" section calling out the net new changes.

#	PR	Title
1	this (#538)	chore(bundle-size): add fixtures for Focusable find* methods
2	#539	chore(deps): patch keyborg to drop unused / legacy code paths
3	#540	refactor(bundle-size): drop dead code paths + declare class fields
4	#541	refactor(bundle-size): use free-function helpers for events and timers
5	#542	refactor(bundle-size): convert Subscribable family to factories
6	#543	refactor(bundle-size): convert DummyInput family to factories
7	#544	refactor(bundle-size): convert API classes to factory functions
8	#545	refactor(bundle-size): extract context resolvers + slim TabsterCore surface
9	#546	refactor(bundle-size): extract Focusable mover/groupper resolver
10	#547	refactor(bundle-size)!: hoist focusedElement helpers to module-level for tree-shaking
11	#548	refactor(bundle-size): extract dummy infrastructure + auto-install via getRootDummyInputs
12	#549	refactor(bundle-size)!: extract Tab keydown handler into opt-in module
13	#550	refactor(bundle-size)!: drop UncontrolledAPI wrapper, store callback directly
14	#551	refactor!: drop @deprecated API surface (keeps noDirectAriaHidden)

After this PR lands on `master`, the remaining stack PRs need to be rebased onto `master` in turn. Doing so once #538 merges keeps each subsequent PR's diff focused on its own net change.

The final state at the tip of #551 matches the original `refactor/api-factories` branch (HEAD `5f758a8`) byte-for-byte, except for the bundle-size analyzer tooling under `scripts/` which is intentionally not shipped.