fix: scope basis unit conversion to touched atom(s) (spec §5.2)

[timurbazhirov]

Summary

• commitMovedAtoms_/addAtom/cloneSelectedAtoms placed a touched atom's coordinate by flipping the whole basis to Cartesian and back (basis.toCartesian()/toCrystal()) around the mutation. Basis.toCartesian/toCrystal round-trip every atom's coordinate via mapArrayInPlace, not just the touched one — so spec §5.2's "untouched atoms preserved bit-for-bit because they are never re-derived" claim didn't fully hold in the implementation, even though it was invisible in practice (Basis rounds every coordinate to Cell.roundPrecision, 9 decimals, on serialize, which absorbs a single round-trip's float noise for any reasonably-conditioned cell — verified directly against @mat3ra/made, including deliberately ill-conditioned lattices).
• Converts only the touched point(s) directly via basis.cell.convertPointToCrystal/convertPointToCartesian instead, at all three call sites. Untouched atoms' coordinate entries are now genuinely never read or rewritten, and each edit does O(touched) matrix work instead of O(n).
• Third PR in the post-review chained-PR stack (stacked on #207); see plan/interactive-editor-spec-plan.md's "Chained PR stack" section for the full writeup.

Test plan

• New describe block "Basis unit-conversion scoped to touched atoms (spec §5.2)" in tests/__tests__/mixins/interactive_structure_editor.js: 3 tests spy on the ephemeral basis instance applyBasisDelta_ builds per commit (scoped to that instance, not the shared Basis prototype, since the wave legitimately calls toCartesian/toCrystal elsewhere for unrelated reasons) proving no whole-basis round-trip happens; 3 more assert untouched atoms' coordinates stay exactly unchanged and a moved atom's committed crystal coordinate round-trips to its exact dragged Cartesian position.
• All 3 "does not round-trip" tests confirmed failing against the pre-fix code and passing after (verified via a temporary git stash of just the source change).
• Full Jest suite green (181 passed, 2 skipped, up from 175).
• tsc --noEmit clean.
• npm run lint clean (0 errors; only pre-existing no-explicit-any warnings matching this file's established convention).
• npm run build clean.

🤖 Generated with Claude Code

[timurbazhirov]

Closing — consolidated into #216 (then #214 on top).

Verified contained: this PR's head baa03bba is a direct ancestor of claude/uiux-p2-figure-export. Spec §5.2's scoped unit conversion is also the reason U-6's units toggle changes the display only rather than converting the material — the two decisions sit next to each other now.

The branch fix/scope-basis-unit-conversion-to-touched-atoms stays on the remote.

---

Generated by Claude Code