fix(v3.1): Phase 54.5 truth leaks + pin rustc 1.97

[RichardHightower]

Why

Phase 54's public claims were true. A review of the wiring found second-order honesty bugs in the explainability payload, a hole in LLM rerank (empty event previews + salience undoing the model order), and two landmines that would contaminate Phase 55 measurements. Main is also red on Clippy because CI tracks floating stable (Rust 1.98 result_large_err on generated tonic stubs).

This is the cleanup the review asked for before more measurement.

What changed

CI / toolchain

• rust-toolchain.toml pins 1.97 so a new stable lint cannot redden main overnight
• CI / e2e-cli / release use dtolnay/rust-toolchain@master so the pin is honored
• #![allow(clippy::result_large_err)] on the include_proto! module (we do not control generated Result<_, tonic::Status> signatures)

Explainability tells the truth

• LLM fail-open (completer error or unparseable order) reports rerank=heuristic
• layers_attempted only lists layers that were supported and invoked
• stop_conditions and mode_override are forwarded into query_ranked_with
• unknown rerank_mode is InvalidArgument; CLI clap-rejects too

LLM rerank actually reranks

• BM25 text field is now TEXT | STORED (events have empty keywords; the reranker was judging empty bodies)
• RankingConfig.preserve_order after a successful LLM rerank so salience cannot undo the paid-for order

Don't plant Phase 55 landmines

• dropped per-event full grip scan on BM25 outbox drain (find_grip_for_event)
• fusion fan-out is concurrent (join_all) unless mode is Sequential
• sequential timeout breaks the whole fan-out, not just the inner query loop
• query path, prune job, and dedup share one HNSW Arc<RwLock<_>>
• dimension comes from the embedder / EMBEDDING_DIM, not a magic 384

Finding to fix

Theme	Fix
Failed LLM rerank still reported rerank=llm	Reranker::mode_name() + fell_back flag
layers_attempted listed layers that were never invoked	filter by executor.supports
stop_conditions / mode_override echoed, not honored	query_ranked_with
Unknown --rerank silently became heuristic	clap value_parser + gRPC InvalidArgument
Event hits had empty text_preview	store text; searcher returns truncated body
Salience re-sort undid LLM order	preserve_order
find_grip_for_event O(n) per outbox entry	removed from BM25 drain
Sequential fan-out summed latencies	join_all unless Sequential
Query HNSW handle diverged from prune	open indexes first; share index_handle()
Hardcoded dimension 384	embedder.info().dimension / EMBEDDING_DIM
Clippy result_large_err on tonic stubs	allow on generated module + pin 1.97

Tests (local)

Crate	Result
memory-orchestrator	34 passed (incl. parallel fan-out, sequential timeout, fail-open mode_name)
memory-retrieval	78 passed (preserve_order)
memory-search	54 passed (event text preview; text is STORED)
memory-service	125 passed (unknown rerank rejected; LLM fail-open reports heuristic)
memory-indexing	48 passed
memory-daemon	93 passed
memory-cli	63 passed (unknown --rerank rejected)
memory-vector	28 passed
clippy -D warnings	green
rustfmt --check	green

Existing BM25 indexes created before this change do not store text and need a rebuild for event previews. New indexes get it automatically.

Note on Phase 56 (PR #34)

PR #34 is independently MERGEABLE; its Clippy failure is the same 1.98 result_large_err drift this PR retires. Merge this first, then rebase #34.