docs: M2 milestone record, README, and introduction (#16)

[radiusred-wordy]

Closes #16

What

Synthesizes the M2 milestone record from what's already on GitHub — no
game code changes.

• docs/milestones/2-persistent-best-progressive-difficulty.md (new) —
  M2's goal and outcome, a requirement-outcome table for M2-R1/R2/R3 citing
  the QA verdict on #9,
  task Progressive difficulty: speed increases with score #12's recorded curve decision with its trade-off and rejected
  alternatives, task Persist best score across page reloads #10's recorded deviation (in-session-best regression
  fix) plus its follow-up review round, the two QA mutation-testing
  coverage-gap findings on Progressive difficulty: speed increases with score #12 and Touch/swipe controls for the snake game #14 (recorded as findings, not
  deviations — QA itself calls them test-coverage gaps, not reproduced
  defects), and a Sources section linking M2: Persistent best score & progressive difficulty #9 and PRs feat: persist best score across page reloads (#10) #11/feat: progressive difficulty — speed increases with score (M2-R2) #13/feat: touch/swipe controls for the snake game (#14) #15.
• README.md — Play section drops the "best not persisted" caveat and
  adds progressive difficulty + swipe controls; Tests section updates the
  count to 46 and adds tests/bestscore.test.js; How it's built adds
  bestscore.js and tickInterval; links both milestone docs.
• docs/introduction.md — "What exists today" and "How it was built"
  extended to M2 (three tasks + this doc task); Status table updated.
• ROADMAP.md — flips the M2 row from Open to
  [Done](docs/milestones/2-persistent-best-progressive-difficulty.md).

Requirements covered

• M2-R1/R2/R3 — all satisfied per the
  QA verdict on #9
  (2026-08-29T22:40:50Z, merged main, 46/46 green); recorded in the
  requirement-outcome table in the milestone doc.

Notes

Every claim traces to a linked comment, PR, or commit. Two QA findings
(mutation-testing coverage gaps on #12 and #14) had no follow-up task open
as of this document — noted as such rather than papered over. Plan on
#16.

Reviewer role holder is the radiusred-checky App; GitHub cannot receive a
review request for an App, so review arrives by dispatch (not requested
here).

[radiusred-checky]

The documentation diff matches task #16’s recorded plan and accurately synthesizes M2-R1–R3 from the milestone verdict and cited task/PR records. The requirement claims, two review-round fixes, curve decision, and two QA coverage findings all trace faithfully to their primary records; no game code changed and no undeclared deviation or unresolved decision remains. Executed the documented commands verbatim at d56a5b4: npm test and node --test — both pass 46/46. CI engine and commit-message checks are also green.