fix(features): gate 6 items on streamable-http/async + add lint-ci-no-default regression guard (Phase 8e, closes #295)

[githubrobbi]

Summary

Closes #295. Companion to the closed Phase-8 issue #195.

Post-Phase-8 audit found that cargo check --workspace --no-default-features (playbook §996, the second of the four required Phase 8 validations) was never run during Phase 8. Doing so reveals 6 items reachable only when feature X is on but not themselves #[cfg(feature = "X")]-gated. Under strict CI (-D warnings) all 6 hard-fail. CI today runs only --all-features configurations which mask the dead-code + unfulfilled-expect warnings these gaps emit.

Fixes

Source (3 surgical cfg/cfg_attr annotations, zero behavior change)

Site	Before	After	Why
uffs-mcp::McpStats::avg_tool_latency_us	always defined	#[cfg(feature = "streamable-http")]	only called by http.rs::/status
uffs-mcp::McpStats::to_json	always defined	#[cfg(feature = "streamable-http")]	only called by http.rs::/status
uffs-client::daemon_ctl::keepalive_send_blocking	always defined	#[cfg(feature = "async")]	only called by connect_keepalive (itself async-gated)
uffs-mcp::lib.rs::run_mcp_server_with_config #[expect(cognitive_complexity)]	always present	#[cfg_attr(feature = "streamable-http", expect(…))]	score only crosses threshold when feature on
uffs-mcp::handler/mod.rs::on_roots_list_changed same	same	same	same
uffs-mcp::handler/mod.rs::call_tool same	same	same	same

Regression guard (new lint-ci-no-default gate)

• scripts/ci/gates.toml: new [[gate]] block; mirrors lint-ci in flag stack + -D warnings strictness; differs only in --no-default-features vs --all-features. Tiers: pre-push + pr-fast.
• just/test.just: new lint-ci-no-default recipe.
• scripts/hooks/_lint_pre_push.sh: regenerated via just gen-hooks (auto from manifest).
• .github/workflows/pr-fast.yml: new clippy-no-default job, registered in required.needs: + aggregator table.
• All 4 drift detectors green: gates-drift (27 gates), hooks-drift, workflow-drift, fast-drift.

Together lint-ci (--all-features) + lint-ci-no-default (--no-default-features) establish the additivity invariant: every pub/pub(crate) item reachable only with feature X must compile cleanly both with feature X off AND with all features on.

Documentation

• docs/architecture/code-quality/dependency_policy.md §2: 5-tool table → 6-tool table; new paragraph explaining the additivity invariant.
• docs/architecture/code-quality/dependency_policy.md §10: 8e decisions-log row.
• docs/architecture/code-quality/trait_policy.md:193: Phase 7f TBD → #291 (closes Phase 8 follow-up Bump the cargo group across 1 directory with 2 updates #1).

Verification

Config	Before	After
cargo clippy --workspace --all-features -D warnings	0	0
cargo clippy --workspace --no-default-features -D warnings	6 warnings	0
cargo clippy -p uffs-client --no-default-features -D warnings	1 warning	0
cargo clippy -p uffs-mcp --no-default-features -D warnings	5 warnings	0
cargo clippy -p uffs-cli --features mcp-http-probe -D warnings	1 warning	0

Pre-push gate: full 24-gate run green in 106 s (lint-ci-no-default fires in ~1 s warm).

5-rule contract adherence

• Rule 1 (no suppression hacks): zero #[allow] added. All 6 fixes are minimum-correct #[cfg] annotations expressing actual reachability. The 3 expects become conditional rather than blanket — they preserve the strict check when triggered, no unfulfilled-expect when not.
• Rule 2 (surgical, correct fixes): 3 source files, 9 attribute lines added, zero logic change.
• Rule 3 (preserve behavior + contracts): default + --all-features binary surface byte-identical. All API contracts preserved.
• Rule 4 (improve tests, don't dodge them): new CI gate gives the whole workspace the structural feature-additivity verification it lacked.
• Rule 5 (document & commit well): comprehensive commit message + 2 policy-doc updates + decisions-log entries.

Follow-ups

This closes Phase 8 follow-up #1 (trait_policy.md TBD) and addresses the audit-finding gap (cargo tree --workspace -e features snapshot is local-only baseline; not in scope for this PR).