Move x-demo-lang symbol semantics behind the puzzle module seam #48

Goal

Concentrate x-demo-lang symbol and edit semantics inside the puzzle module so that methods.rs is a pure protocol adapter: it shapes wire params and response envelopes, and asks the language module named questions instead of deriving semantics from AST fields.

Context

An architecture review of the conformance suite (2026-10-06, against b4be4d2 / release 1.5.0) found the deepest leak in the mock provider: roughly 200 LOC in methods.rs is x-demo-lang semantics, not protocol logic.

• symbol_at_position, puzzle_symbols, and rename_edits live in the wire-adapter module.
• The definition, references, and rename handlers each re-derive declaration-vs-reference rules from Puzzle's public fields (name, ops, solution, name_range, arg, ...), so the same language semantics exist as three private derivations.
• puzzle exposes its whole AST — ~20 re-exported names plus public struct fields — so the seam is wide and shallow: callers do the interpreting.
• The moved helpers are reachable only through spawned-process fixtures today, while puzzle/tests.rs is an existing unit-test surface at exactly this seam.
• The mock is the reference implementation third-party provider authors read and copy; "language answers queries, adapter shapes wire" is the structure it should demonstrate.

Scope

• conformance/mock-provider only: methods.rs, puzzle.rs, puzzle/*.
• Move language-aware symbol/edit logic (document-symbol listing, declaration lookup, occurrence collection, rename-target classification and edit planning) behind the puzzle module's interface as typed query results.
• Keep protocol-owned concerns in methods.rs: wire param shapes, response envelopes, and the method-namespaced refusal codes (definition.noSymbolAtPosition, etc.).
• Once no caller derives semantics from AST fields, Puzzle/Op/SolutionEntry fields can stop being public.

Non-goals

• No wire-contract, spec, or fixture changes. Zero protocol-visible behavior change.
• No new x-demo-lang features or methods.
• No changes to the runner or to puzzle's lex/parse correctness work.
• Does not prescribe the exact query names/signatures — that is implementation detail, not contract.

Constraints

• The full conformance fixture suite must pass unchanged: lpp-conformance-runner --provider ./target/release/lpp-mock-provider --fixtures conformance/fixtures/v1.
• cargo test --workspace, cargo clippy --all-targets -- -D warnings, cargo fmt --all --check stay green.

Acceptance criteria

• methods.rs no longer inspects Puzzle/Op/SolutionEntry fields to derive declarations, references, or rename edits; the definition/references/rename/symbols handlers obtain results through puzzle-module queries.
• The moved semantics are covered by unit tests under puzzle/tests.rs, not only by end-to-end fixtures.
• The fixture suite passes with zero fixture changes; refusal codes and response shapes are byte-identical on the wire.
• Deletion test passes: removing any single moved helper no longer spreads x-demo-lang logic across methods.rs.

Dependencies / ownership

• language-provider-protocol owns the reference mock provider; no cross-repository work.

Readiness

Ready for implementation. Internal refactor with an existing verification boundary (unit-test harness plus the unchanged conformance suite); no product, architecture, or contract decision is open.