feat(query): existence-test error surfaces and docs — the feature's edges

[0x054]

Closes #317, closes #311, closes #307, closes #308

Stacked on #321 (slice 3/4); base retargets as the stack merges. Final slice of the existence-tests PRD (#311, ADR-0007) — when this merges, the PRD and the originating Pinch PRDs (#307, #308) close.

Error surfaces

Every remaining place a reverse or M2M relation can be named now answers with the supported spelling:

• join()/left_join() on a reverse/M2M edge stays rejected — the pinned "a join never multiplies root rows" property is preserved by rejection, not implicit DISTINCT — and the error now names .exists() (previously the opaque generic "must return a relation path"). Pinned for reverse-FK, M2M, and both join verbs.
• in_() with a query RHS stays a TypeError, and the message now names the existence test when the RHS is a Query/ProjectedQuery (the feat Reverse-relation membership predicates: EXISTS from the root query (PRD from Pinch) #307 repro's second guess: t.id.in_(subquery)). A non-query RHS keeps the plain message — no hint where none applies. in_(subquery) remains declined, not deferred (ADR-0007).
• include() rejection verified unchanged for M2M edges too — population stays a separate future mechanism; membership filtering and population remain distinct axes.

Docs

• New guide section — "Existence Tests on Reverse & Many-to-Many Relations": real models with Assignment + Annotated tabs, lambda-style predicates throughout, rendered SQL for both the feat Reverse-relation membership predicates: EXISTS from the root query (PRD from Pinch) #307 is_transfer shape and the feat OR-composition across a left-joined reverse child table (PRD from Pinch) #308 line-aware category filter, the explicit grouping contrast (exists(A & B) vs exists(A) & exists(B)), negation, forward traversal inside the subquery, nesting, M2M from both sides, the loud dead ends, and the exists-test vs await query.exists() terminal disambiguation. Backed by two runnable example scripts (existence_tests.py + annotated twin) exercised by the docs-example harness.
• Operators table gains the .exists(...) row; the forward existence/absence section and the instance-side reverse-relation section cross-link the reverse direction.
• Reference (api/queries.md) states the one-verb rule and its rejections.
• Roadmap + "Not Yet Supported" now distinguish reverse predicates (shipped — the existence test) from reverse population (still future — include() rejection stays), and track cross-scope correlation as feat Cross-scope correlation in existence tests (deferred from ADR-0007) #309.

Acceptance criteria (#317)

• left_join() on a reverse/M2M edge raises naming .exists() (pinned by test)
• in_() with a query RHS raises naming .exists() (pinned by test)
• include() on a reverse/M2M edge unchanged (existing rejection verified by test)
• Guide section: existence tests with rendered SQL, grouping contrast, negation, M2M; Assignment + Annotated tabs on model declarations; lambda predicates throughout
• Reference + roadmap/"not yet supported" docs updated (reverse predicates shipped; reverse population still future)
• Docs examples covered by the docs-example test harness

Testing

• pytest --db-backends=sqlite,postgres: 1679 passed (full matrix, local Postgres), including the new pinned error surfaces and both docs example scripts
• ty check and ruff check: clean
• No Rust changes in this slice

[0x054]

Superseded by #323 (slices 2–4 combined into one rebase-mergeable PR; content identical).