spec: pin message-literal source grammar (RFC-001 §5.1)

[trendvidia]

Pins the source-level spelling of message and list literals (#102) — the follow-up from #64/PR #99, which settled the carrier but left the literal production unexpanded.

Grammar

literal        ::= scalarLit | listLiteral | messageLiteral
literalValue   ::= literal | qualifiedIdent          (* enum-value reference *)
listLiteral    ::= "[" (literalValue ("," literalValue)*)? "]"
messageLiteral ::= qualifiedIdent? "{" (fieldInit ("," fieldInit)*)? "}"
fieldInit      ::= Ident ":" literalValue

Canonical spelling: @example(myco.commons.Money{currency: "USD", units: 5}).

Decisions

• Type name follows the §8.1 typing rule: REQUIRED when the param (or, recursively, the initialized field) is any/google.protobuf.Any; OPTIONAL and compiler-verified against the declared type otherwise; never inferred from the value's shape.
• Colon field-inits, tight subset: mandatory : and commas, no trailing comma, each field at most once, unknown field = error. Textformat liberties (colon-less blocks, semicolons, repeated-by-repetition, [type.url]) excluded. Repeated fields take list literals; map fields deferred (compile error in v1.2).
• Enum-typed fields/elements = qualifiedIdent, linker-resolved (EnumLiteral); lists homogeneous, nestable, []/{} legal; no expressions inside literals.

Acceptance criteria (#102)

• literal production expanded (message + list) in RFC-001 §5.1 and docs/grammar.ebnf (new clearly-scoped schema-extension section with PXF-divergence notes)
• IETF draft -01 grammar section updated (ABNF additions + new Literal Values section; resolved "Literal value shape" open-issue bullet removed)
• Fixture: 10_literal_args.proto gains the Money{...} use site — golden 11_literal_carrier_golden.textproto's previously-orphaned middle entry now has its source counterpart; locations synced (49/54/58)

Plus a resolution record under docs/RFC-001-issues.md §#15 (second commit).

Closes #102