refactor(state): align inverse calc naming and collapse activeTab mutation (#692)

[grzanka]

Closes #692.

Two low-risk consistency cleanups in src/lib/state/, no behaviour change.

A. Inverse -calc split convention

The multi-* families follow "data file + a single-effect *-calc.svelte.ts"
(multi-entity + multi-entity-calc, multi-program + multi-program-calc).
Inverse broke this by bundling two unrelated effects into one file.

• Split inverse-calc.svelte.ts into:
  • inverse-range-calc.svelte.ts — setupInverseRangeCalculation
  • inverse-stp-calc.svelte.ts — setupInverseStpCalculation
• Each file now holds exactly one $effect, mirroring the multi-* layout, with
  a doc comment pointing at its sibling.
• Orchestrator imports updated; combined test renamed to
  inverse-calculation.test.ts (it exercises both effects), imports repointed,
  assertions unchanged.

B. Single activeTab mutation path

inverse-lookups.svelte.ts exposed both set activeTab(v) and
setActiveTab(tab). Removed the setter, keeping setActiveTab(tab) as the sole
mutation entry point and marking the interface field readonly. All call sites
(orchestrator, +page.svelte, tests) already used the method, so no call-site
changes were needed.

Acceptance criteria

• Inverse calculation effects follow the documented multi-* convention
  (one effect per *-calc.svelte.ts file, with cross-reference comments).
• activeTab has exactly one mutation entry point (setActiveTab); field
  is readonly in the interface.
• pnpm check (0 errors) and pnpm lint (clean) pass; inverse-lookup
  tests unchanged in behaviour (42 tests pass).

Note: the unrelated guard-forbidden-files.test.ts failures seen locally are a
sandbox git commit-signing limitation, not caused by this change.

https://claude.ai/code/session_01RtN8sSC9RkYV2jM34xKZdz

---

Generated by Claude Code

[Copilot]

Pull request overview

This PR applies two code-health consistency refactors in the calculator “inverse” state layer under src/lib/state/, aligning it with the existing multi-*-calc convention and reducing redundant API surface on activeTab (no intended behavior change).

Changes:

• Split inverse lookup calculation effects into one $effect per *-calc.svelte.ts module (inverse-range-calc and inverse-stp-calc) and update imports (orchestrator + unit tests).
• Collapse InverseLookupState.activeTab mutation to a single public entry point (setActiveTab), making activeTab readonly in the interface.
• Add/update AI session logging (changelog entry + session log + log index row).

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/lib/state/inverse-range-calc.svelte.ts	New dedicated module for the inverse CSDA-range calculation $effect.
src/lib/state/inverse-stp-calc.svelte.ts	Leaves only the inverse stopping-power calculation $effect and documents the sibling module split.
src/lib/state/inverse-calc.svelte.ts	Removed in favor of the one-effect-per-file split.
src/lib/state/inverse-lookups.svelte.ts	Makes activeTab readonly in the interface and removes the redundant set activeTab(...) setter implementation.
src/lib/state/calculator-page-orchestrator.svelte.ts	Repoints setup imports to the new inverse calc modules.
src/tests/unit/inverse-calculation.test.ts	Updates imports to the split inverse calc modules; assertions remain the same.
CHANGELOG-AI.md	Adds the required session entry linking to the detailed AI log.
docs/ai-logs/README.md	Adds the new session log to the index table.
docs/ai-logs/2026-06-04-issue-692-state-naming.md	Adds the detailed AI session narrative + task record for #692.