perf: back calculator getters with $derived (#689)

[grzanka]

Closes #689.

What

The public getters rows, stpDisplayUnit, and validationSummary in src/lib/state/calculator.svelte.ts each called a compute*() / get*() function on every property access, rather than exposing memoized $derived values:

get rows() { return computeRows(); }                 // re-parses every row, every read
get stpDisplayUnit() { return getStpDisplayUnit(); }
get validationSummary() { return computeValidationSummary(); }

Templates read these several times per frame (table body, footer, validation banner, export wiring), so the per-row parse/convert/result-lookup work ran multiple times for a single render, and each read handed back a fresh object — a latent identity hazard for any === / capture-and-compare call site.

Change

• Introduced module-level $derived values, matching the existing pattern in entity-availability.svelte.ts:
  • const rows = $derived(computeRows())
  • const stpDisplayUnit = $derived(getStpDisplayUnit())
  • const validationSummary = $derived.by(...) — now derives from the memoized rows value instead of re-running computeRows(), so one parse pass backs both the row view and the validation counts.
• Public getter surface is unchanged (each getter returns its derived value), so no call sites change.
• Confirmed the compute* / get* helpers are pure reads of reactive state (no mutation), so no untangling was needed.

Acceptance criteria

• rows, validationSummary, stpDisplayUnit are backed by $derived.
• Reading a getter twice without an input change returns the same memoized value (added a focused referential-stability test, plus a complementary test asserting the reference is invalidated after an input change).
• No behavioural change in calculator output; existing calculator-state.test.ts passes unchanged.
• pnpm check (0 errors), pnpm lint (clean), calculator-state.test.ts 59/59 pass.

Notes

The only full-suite failures are in guard-forbidden-files.test.ts, which fail because git commit signing is unavailable in the CI sandbox (the signing server returns HTTP 400 during the test's makeRepo). They are unrelated to this change.

https://claude.ai/code/session_01Tvdg51J44XeNxiwP9bnEjS

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Generated by Claude Code

[Copilot]

Pull request overview

This PR improves calculator render-time performance and correctness-by-identity by memoizing hot public getters in createCalculatorState() with Svelte 5 $derived, avoiding repeated per-row parsing/conversion on each property access.

Changes:

• Backed rows, stpDisplayUnit, and validationSummary getters with memoized $derived / $derived.by values (with validationSummary derived from the memoized rows).
• Added unit tests asserting referential stability across repeated reads and invalidation after an input change.
• Logged the AI session (changelog entry + detailed session log) per repo process.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/lib/state/calculator.svelte.ts	Replaces recompute-on-access getters with $derived-memoized views; makes validationSummary depend on memoized rows to avoid duplicate row parsing.
src/tests/unit/calculator-state.test.ts	Adds focused tests for derived getter memoization (stable references) and invalidation on input updates.
docs/ai-logs/README.md	Adds the new session log entry to the index table.
docs/ai-logs/2026-06-04-issue-689-derived-getters.md	Adds the detailed AI session narrative and task record for Issue #689.
CHANGELOG-AI.md	Prepends a new bullet entry for this session with a link to the detailed log.