S2 0.3.0: a single-document answer may describe a workspace in part

[speak-agent]

Summary

S2 0.3.0: in single-document mode, a document may carry data together with error diagnostics, and it then describes everything except what those diagnostics name.

• data (S2-3.4-5) is present when the command described the workspace in whole or in part. A command without data has still failed (S2-3.4-11, unchanged).
• A diagnostic may carry path (optional), the file it concerns, relative to the workspace root. The schema gains the property.
• S2-3.4-12, S2-3.4-13: the producer names each part it could not describe (a workspace member or a package) in an error diagnostic whose path is that part's build description file; a consumer should use the rest of the document and report the errors; the exit status is non-zero.

Why

mcpp-community/mcpp#699: on a five-member workspace, one member whose host tool failed to build removed every member's sets, and the language server fell back to a guessed model. mcpp now plans every selected member on its own and describes a package whose build program fails without that program's directives (mcpp-community/mcpp#702, design record .agents/docs/2026-09-26-compile-database-and-issue-699-design.md §4.4). S2 0.2.0 had no partial outcome, so the producer's new shape needed one sentence here.

Compatibility

MINOR: an optional field and a relaxation of when data appears. A consumer written for 0.2.0 reads such a document as a success with error diagnostics, which is the intended reading.

[Sunrisepeak]

Folded into #26 (release 0.0.5) so it lands as one PR: the S2 0.3.0 change is merged there unchanged, together with its consumer side in mcppls (F10: a partial answer is used and each part the producer could not describe is reported as producer-partial), the traceability evidence S2-3.4-12/13 needs (validate.py checks and fixture mcpp-emit-partial), and the specs CHANGELOG entry. Closing in favour of #26.