Trigram index has no file limit: a 6.5M-file non-git root took 16 min and about 9 GB #1007

Problem

The trigram (search) index has no limit on the number of files. When a session starts in a very large directory that is not a git repository, AFT builds a trigram index of all files in it. In our case this one build took 16 minutes and increased the module's committed memory by about 9 GB. Other roots on the same module did not change.

Evidence (aft 0.62.0, subc module)

• A session started with its directory set to a shared scratch directory under /tmp (147 GB, not a git repository, no project markers). AFT bound that directory as the project root.

• Log:

  • index_event kind=build_started plane=search … root=<scratch dir> at 19:45:29Z
  • index_event kind=build_ready plane=search … root=<scratch dir> elapsed_ms=969598 files=6509755 trigrams=651600 at 20:01:38Z

• Committed memory of the module (Anonymous + Swap from /proc/<pid>/smaps_rollup), with 14 open roots in all samples:

  Time (UTC)	Committed
  19:40:52	3.60 GB
  20:10:52	13.0 GB
  20:41:04	13.0 GB
  21:11:07	13.0 GB

• The root was quiesced at 20:02:40Z (quiesced unbound root … cause=goodbye_unbound). Committed memory stayed at about 13 GB until an external memory guard restarted the module at 21:18Z. We did not find whether the index data or allocator retention kept the memory.

• The semantic index refused the same root (more than 20000 files exceeds limit of 20000). The trigram build has no equivalent check.

Code (main abdc5b4cc)

• The only build-time file limit is semantic.max_files (crates/aft/src/config.rs:300-307, default 20,000 at config.rs:367).
• project_root_is_too_broad (crates/aft/src/commands/configure.rs:678-697) disables indexing only for path shapes (volume root, account home, their ancestors). It does not look at size, so a large directory below the home or under /tmp gets a full trigram build.
• The max_files values in search_index.rs (for example GrepExaminationLimits at search_index.rs:1415-1420) limit queries, not the build.

Steps to reproduce

1. Make a directory that is not a git repository and has a few million small files (for example, many copies of a node_modules tree).
2. Start a session with this directory as its working directory, so that AFT binds it as the project root.
3. Look for the build_ready plane=search line and the module's memory during and after the build.

Expected

A limit on the trigram build, the same as for the semantic index. For example, a search file limit with a clear warning such as "trigram index skipped: more than N files", or the root_too_broad behaviour (explicit-path tools stay available) when the walk passes the limit. The memory that the build uses should also go back after the root is quiesced or evicted.