build(t27c): put the web stack behind cargo features (212 -> 49 crates)

[gHashTag]

Closes #5912. Part of #5905 (weak point 5, t27c build time). Stacked on #5900; retarget to master after #5900 merges.

What changes

cargo build -p t27c stops building an HTTP client, TLS, an async runtime, a web framework and a JWT library. Only serve, bench-endpoints, bridge and audio use them.

feature	what it switches on	used by
(default)	the compiler only	parse, typecheck, gen, suite, seal, ...
net	reqwest	bridge, audio
server	net + axum, tokio, hyper, hyper-util, http-body-util, tower-http, tokio-stream, futures-util, jsonwebtoken, serde_urlencoded	serve, bench-endpoints

• uuid, tower and ignore were not imported by any file and are removed.
• The HTTP half of bridge.rs moves to bridge/client.rs under cfg(feature = "net"). bridge handoff reads a local file, so it stays in every build.
• Without net, the other bridge subcommands and audio exit 1 and print the rebuild command (--features net). They do not vanish from --help.
• jsonwebtoken, hyper and serde_urlencoded are also dev-dependencies, so the jwt::tests and proxy::tests unit tests still run under a plain cargo test (proxy.rs tests are stripped before type-checking — no workflow passes --features server #2301).
• The Dockerfile already builds --features server and is unchanged.

Measured

cargo tree -p t27c -e normal,build --prefix none | sort -u | wc -l:

build	crates
base (#5900), default	212
this PR, default	49
this PR, --features net	145
this PR, --features server	215

cargo check --release -p t27c passes for each of the three configurations.

cargo test -p t27c --no-fail-fast, default features: 2789 passed, 2 failed, the same result as the base commit e82a5b4e4. Both failures are already red there and do not involve these crates:

• each_backend_artefact_write_is_guarded_at_its_own_call_site (tests/corpus_unresolved.rs:309)
• corpus_classifier_matches_lean_completeness (icarus_lowerable)

The six jwt::tests / proxy::tests tests run and pass.

Behaviour without net:

$ t27c bridge status
Error: 'bridge' requires 'net' feature (only 'bridge handoff' is offline); rebuild with: cargo build --release -p t27c --features net
(exit 1)
$ t27c audio --token x
Error: 'audio' requires 'net' feature (rebuild with: cargo build --release -p t27c --features net)
(exit 1)

🤖 Generated with Claude Code

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