Spec: pin ordering, field-handling, and error-coverage clarifications #81

Audit findings M13 + M15 + M18 + L8 + L9 + L10 + L12 + L13 + L14 + L15 + N14 + N15 — spec-verified.

Problem — underspecified or self-contradictory spec text

• M13 §19.1 "MUST ignore unknown fields" vs §4.1 -32602: clarify that a defined field used in an inapplicable context (e.g. entry on lpp/symbols) is -32602, while genuinely unknown fields are ignored — matching the mock.
• M15 §10.2 "MUST advertise the sourceIdentity capability" contradicts capability independence/absent≡false (fixture 44 pins false). Reword to "the capability is defined from 1.3".
• M18 §9.2 result ordering ("request-observed order" vs "mock orders by URI") is self-contradictory; pin URI order (or explicitly provider-defined deterministic order). Same gap for symbols and rename result ordering.
• L8 §16.3 overlappingEdits "the later edit in the original request order" is ambiguous; mock reports the later-in-document-order edit's index.
• L9 §14.2 "sorted by range.start" + "declaration first" conflict for forward-referencing languages; pin the declaration-first carve-out explicitly.
• L10 invalidPosition is only named for lpp/definition; mock applies it to all position-taking methods — extend the spec.
• L12 DocumentSet key vs inner Document.uri authority is unspecified; mock treats the map key as the lookup key and the inner uri as identity — pin that or require equality.
• L13 Post-shutdown responses have no defined error; pin one or forbid responding.
• L14 languageList marker asserts non-empty languages + 2-key entries beyond the §7.2 type contract; align one side.
• L15 Whitespace-only lines and the error-precedence pipeline are undocumented.
• N14 §18.3 invalidLanguage wording covers only document-level languageId, but §21/§22 raise it for the top-level param too.
• N15 §16.3 equal-start zero-length edits: order undefined.

Acceptance criteria

• Each item gets normative text matching mock + fixture behavior (or an explicit provider-defined marker).
• Review-based acceptance; suite green.