## What
Scope 3 of the pluggable-compressor system: a **selected external
`headroom.compressor` plugin now compresses real traffic**. Opt-in via
`--compressor` / `HEADROOM_COMPRESSORS` — external (non-built-in) names
flow to `ContentRouterConfig.active_external_compressors`, resolved once
against the registry in `__init__` (built-in inventory entries filtered
out).
## How
A single guarded branch at the top of `_apply_strategy_to_content`,
immediately before the built-in if/elif. When a selected external
compressor declares the block's detected content type (exact MIME,
`text/*`, or `*` wildcard), the block runs through the pure-data
`Compressor` contract; otherwise it falls through to the built-in path
unchanged.
## Cache safety (by construction)
The branch lives **inside the per-block strategy dispatch**, which only
runs on non-frozen, already-compressible blocks — the frozen/cached
prefix is split off upstream in `apply()`. So a selected external
compressor **can never rewrite cached-prefix content and bust the prompt
cache**; it inherits the exact same cache-preservation the built-ins
have.
## Fail-open + fidelity
Raise, malformed/non-`CompressOutput`, empty-from-non-empty, or
expansion all fall back to the built-in path. Tokens are counted with
the router's own estimator (never the compressor's self-report). Any
`recoverable` (hash→original) map is mirrored to the CCR store like
SmartCrusher, so `/v1/retrieve/{hash}` resolves. Reached only in
lossy/CCR mode (lossless-only sessions return earlier), so it can't
inject unrecoverable loss.
## Behavior change
**None by default.** With no external compressor selected, the branch is
a single cheap guard and everything below is byte-identical to today.
## Testing
`tests/test_router_external_dispatch.py` — end-to-end dispatch of a
selected external compressor, recoverable-map retrievability,
non-hex-hash skip, fail-open on raise/malformed/empty/expansion,
not-selected & non-matching-content-type leave the built-in path
unchanged, wildcard selection. Offline suite: 84 passed (this file +
selection + registry + settings_store). ruff + mypy clean.
Note: the broad content-router/compression suite exercises real HF-Hub
model downloads + local ONNX inference and is slow/flaky in some local
envs — deferred to CI.
Stacks on #2370/#2371/#2373 (all merged).
