docs(guide): long-form GUIDE.md with scrut-tested examples

[srnnkls]

What

Adds GUIDE.md, the long-form companion to the README, plus a scrut suite that keeps its worked examples honest.

The guide is ordered features-and-DX first, internals-and-design last:

• Front half (authoring DX): how fas works (patterns-not-predicates mental model), your first rule end to end, the stdlib vocabulary table, writing when (subsumption, the parsed view, structural negation, what when can't express), the five then verbs, how decisions combine, global/project layering, package organization, and the explain/vet/diagnostics loop.
• Back half (internals): an "Under the hood" section (subsumption engine, preprocessor bridge, closed-world matching, embedded stdlib module overlay, two-phase pipeline, synthesis, localize→render diagnostics, fail-open) and a "Design motivations" section.

Tests

tests/guide.md is a scrut suite (mirroring tests/policies.md / tests/diagnostics.md) asserting every worked example byte-for-byte, with fixtures under tests/guide_rules/ (project layer) and tests/guide_rules_global/ (global layer, for the layering examples). It's picked up automatically by mise run test-integration (glob tests/*.md).

Every output in the guide was captured from a real build, not written from memory.

• scrut test -w . tests/*.md → 88/88 pass (12 new, no regressions)
• go test ./tests/ → pass (the coverage/drift Go tests scan only tests/policies/, so the new fixture dirs don't perturb them)

Notes

• The guide describes the diagnostic surface accurately rather than over-promising: composed-matcher misses (tool.#Bash & list.MatchN(...)) don't emit a caret frame — only the leaf/kind/disjunction shapes in tests/diagnostics.md do — so the guide's explain test asserts exit codes (0 fired / 1 missed), which are deterministic.
• "Wiring into Claude Code" describes hook registration in prose but omits a concrete settings.json snippet, since that schema is Claude Code's, not fas's. Easy to add if wanted.

[srnnkls]

Updated: rebased onto feat/stdlib-didyoumean-suggestions (was main) — main predates fas vet, FAS_LOG, and the additive-injects fix that the guide documents, so the scrut suite only passes against the feature binary. Will retarget to main once the feature branch lands.

Also added: each subsumption footgun in the guide now names a loadable alternative, backed by tests/guide_rules_alts/ and a new scrut section (93/93 pass).