fix(pipeline): size definition properties for every field - #2687

What

build_def_props (in pass_definitions.c and its twin in pass_parallel.c) appends a definition's fields atomically: a field that does not fit is dropped whole. The buffer was sized only for the docstring, variants and variant_key, and every other field shared the remaining 2 KB.

On wide definitions (long signatures, decorator lists, parameter lists), that left no room for the three fields appended last:

• fp: the MinHash fingerprint;
• sp: the structural profile;
• bt: the body tokens.

They were dropped silently. Such definitions got no SIMILAR_TO edge and fewer SEMANTICALLY_RELATED edges. Any newly added property pushed more of them out; #1692's route_path / route_method dropped fp on 12 long-docstring jellyfin controller actions and cost 34 SIMILAR_TO edges.

pd_props_buf / pp_props_buf (twins, kept in lockstep) now count:

• the fixed metrics prefix;
• the whole serialized form of every field build_def_props appends;
• the reserve that the fp / sp / bt room checks use.

Definitions that fit keep the stack buffer. The field order is unchanged, so definitions that already fit produce byte-identical properties.

Consumers of these properties are pass_similarity, pass_semantic_edges and semantic search; MCP's search_graph already filters fp / sp / bt out of agent output. None of them depends on the fields being absent, so this is not an index format change.

Evidence

Corpus run against current main (354ccec), all PHP and C# corpus repos (PHP 27 par / 26 seq, C# 29 par / 29 seq; the heavy roslyn/runtime pair excluded):

• No other edge type moves on either venue.
• 324 of the 325 removed SIMILAR_TO edges are displacements under the per-node edge cap: one endpoint gained a better-ranked neighbour once more definitions carried a fingerprint. The remaining one fits the LSH candidate cap.
• SEMANTICALLY_RELATED is TF-IDF with corpus-wide document frequencies plus a threshold. Recovered body tokens shift IDF across the repo, so some near-threshold pairs cross either way.

Test: parallel_wide_definition_keeps_fp_sp_bt_f14, both resolvers.

• A Python function with 50 annotated parameters loses fp, sp and bt without the fix (props 1,904 B) and keeps all three with it (2,600 B).
• A control with the same body, small, has all three either way (1,026 B, unchanged).

Suites run locally: parallel, pipeline (+ conventions / frozen / semantic-manifest), semantic, incremental, store_search: 819 passed. cppcheck 2.20 (the CI version) and clang-format are clean on the changed files.