Migration 40 dropped the symbol tables and graph edges, but the daemon
could still adopt an external symbol_extractor_v1 plugin (for example a
0.19 plugin left in a user plugin directory) and rebuild them.
Remove the interface header, the ABI adapter, plugin adoption, the
EntityGraphService extract-and-write path, the post-ingest symbol
channel, the write-coordinator symbol ops, and the KG-store symbol
metadata and extraction-state API. The ABI loader and the external host
now refuse a plugin that declares the interface (or the legacy
symbol_extraction capability) with a one-time warning. Status no longer
reports symbol extractors, generated configs no longer write
[plugins.symbol_extraction] (an existing section still parses), and
the benchmark suite drops symbol_extraction_bench.
Tests that exercised the dropped tables or the v39 schema template are
removed; the rest run against the current schema.
BREAKING CHANGE: code-symbol extraction is gone. Removed: the
symbol_extractor_v1 plugin ABI, `yams config grammar` (list, download,
path, auto-enable, auto-disable), the grammar steps in `yams init`, the
meson option `plugin-symbols`, the conan option
`enable_symbol_extraction`, and the Windows installer SymbolPlugin
component. Databases migrate to schema 41; a v0.19 binary refuses to
open them, and symbol data dropped by migration 40 cannot be recovered.
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