chore: simplify review follow-ups (memoize sort, cache npm, trim comments)

[heznpc]

Summary

Three targeted wins from `/simplify`. Net +19/-23 lines. Each item only landed after triaging for value — the review also flagged a CI composite-action extraction and a selector flatten, both skipped as over-engineering vs. real cost.

1. Pre-sort `FLASHCARD_COURSE_MAP` once at module load

`Object.entries(map).sort((a,b) => b[0].length - a[0].length)` was running inside the per-URL course resolver in both content.js:320 and sidebar-chat.js:868. Map has ~30 entries so it's not a real hot path, but a pre-sorted `FLASHCARD_COURSE_SLUGS_SORTED` constant is strictly simpler than recomputing. Added to the ESLint global list alongside `FLASHCARD_COURSE_MAP`.

2. Enable npm cache in CI

`validate`, `build`, `test` all run `npm ci` in parallel. With `cache: 'npm'` on `actions/setup-node@v4`, the 2nd and 3rd jobs hit the warm cache and skip the cold install. Expected saving ~60–120s per PR run.

3. Tighten two comment blocks

• selectors.js "defensive selectors" preamble said the same thing three times — compressed to 3 lines.
• constants.js `FLASHCARD_COURSE_MAP` docblock over-explained the longest-match semantics; trimmed to 6 lines with an explicit note that keys are hyphenated slugs, values are camelCase deck names.

Findings that were deliberately skipped

Finding	Why skipped
Extract CI setup into composite action / reusable workflow	`cache: 'npm'` solves the actual cost (cold installs) with one line; composite action adds a file to maintain for the same payoff
Flatten `li.lesson-modular div.title, .lesson-row div.title, ...` into separate array entries	`querySelectorAll` handles comma-alternates fine; speculative hot-path, no evidence of real cost
Extract a `lessonRowVariants()` helper for 2 branches	Single line, overkill
Rename CI step "DOM health check (Skilljar selectors)"	Cosmetic
Reword "conservative" in selector comment	Already trimmed out in the main compression

Verification (local)

• Tests: 309/309 pass
• Lint / format / selector health / dicts / bg-sync / glossary / translate validate — all green
• `npm run build:firefox` + `build:bundle` — both build clean

Test plan

• CI: `validate` + `test` + `build` jobs all pass and show cache-hit logs from `actions/setup-node`

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