spec: RFC-001 contextual keywords (no soft-break for v1.2)

[trendvidia]

Summary

Amends RFC-001 to make type, function, and annotation contextual keywords — recognized only at the start of a top-level declaration; accepted as identifiers everywhere else. Removes the soft-break that the original v1.2 reservation would have introduced.

Also drops the spurious reservations of expression (only meaningful in annotation-parameter-type position) and this (only inside engine-language bodies, which protocompile captures opaquely). Reduces the touched-tokens count from 5 to 3.

Follow-up to #54 (RFC corpus) and #70 (M1 kickoff doc). Tracked by #55.

What surfaced this

The M1 implementation pre-flight surfaced oneof type { ... } in Google's Cloud DLP API schema (Apache 2.0, vendored at parser/testdata/largeproto.proto in protocompile, 8 occurrences). The pattern is common across production protobuf schemas in the wild, including Google's own gRPC APIs.

Hard-reserving the words would break wide swaths of real-world protobuf code. Contextual recognition preserves every v1.1 schema unchanged — the parser uses the file-scope production's lookahead to distinguish type Email = ... (keyword) from oneof type { ... } (identifier).

Pattern is well-precedented: Java 9 added module, requires, exports, opens, to, with as contextual keywords for the same reason.

What this PR changes

File	Change
docs/RFC-001-schema-extensions.md	§5.1 — contextual-keyword design + rationale; explicit non-reservation of expression and this
docs/draft-trendvidia-protowire-01.md	Reserved Keywords section + appendix ABNF — contextual-keyword wording
STABILITY.md	v1.2 schema-language section — "no source-level incompatibility introduced in v1.2"
CHANGELOG.md	[Unreleased] entry — contextual-keyword note
docs/M1-kickoff-protocompile.md	PR 1 scope adjusted to 3 keywords + a contextual-keyword grammar bridge; identifier-audit replaces soft-break-audit; estimate revised 1–2 days → 2–3 days

Backward compatibility

After this amendment, the wire and semantic compatibility statements simplify:

• Wire format — unchanged (was already)
• Source format — also unchanged; no schemas need renaming for v1.2

The migration story moves from "rename any identifier that collides" to "nothing changes."

Test plan

• Read pass — confirm contextual-keyword design is consistent across RFC, IETF, STABILITY, CHANGELOG, kickoff doc
• Audit grep — confirm the proposed grammar bridge (RFC §5.1 + kickoff §"What changes" item 3) covers all _NAME name-binding positions in protocompile/parser/proto.y
• Confirm no other open issue (Ratify RFC-001 #56–Upstream buf/protocompile compatibility strategy #69) needs updating in light of the amendment