feat: markdown → scenario-data generator (tools/)

Add a deterministic authoring tool that turns a structured scenario
markdown file into a validated scenario-data module:
- tools/scenario-md.js: pure parseScenarioMarkdown + emitScenarioModule
  (frontmatter → meta; ## Timeline/## Clues/## Cast lists → the arrays;
  auto-slugged ids; validated via validateScenario before emit).
- tools/md-to-scenario.mjs: CLI (npm run gen:scenario), file or stdout.
- tools/example-scenario.md: documented format example.

Parses a defined format, not freeform con prose. tools/ is dev-only and
never bundled. tests/tools/scenario-md.test.js round-trips + validates.
187 tests pass; build clean.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tools/example-scenario.md

@@ -0,0 +1,36 @@
+---
+id: example-md
+title: EXAMPLE — Generated From Markdown
+system: year-zero
+players: 4
+playMinutes: 210
+slot: Fri · Slot 2
+---
+
+# Notes are ignored — only frontmatter and the sections below are parsed.
+
+## Timeline
+
+Beats are `[minutes-from-start] label`. Add `{hard}` for a hard trigger,
+`| cut: …` for a cut hint, and `#id` to pin a stable id (else it is slugged).
+
+- [0] The opening beat {hard} #a1-open
+- [35] Parlor interview | cut: summarise the ledger; skip the tea ritual
+- [120] The reveal {hard}
+- [200] Ambiguous epilogue
+
+## Clues
+
+`{essential}` marks a clue the ending needs; `{act: …}` tags the act;
+`| fallback: …` is how to deliver it if missed.
+
+- The door-cam face that cannot exist {essential} {act: Act One} | fallback: the monologue cards
+- The identical childhood, recited twice {essential}
+- A colour detail {act: Act One}
+
+## Cast
+
+`NAME — one-line note`, optionally `| secret: …`, and `{pc}` for a player character.
+
+- MARY FLETCHER — Innkeeper; saw the watchers | secret: she saw Crowe sew the pendant
+- DANNY TATE — Stable boy, 14