Add FirstPerson Model first-person body to ray tracing - #2

FirstPerson Model renders a complete first-person body state that vanilla's
pipeline never feeds into Caustica's ray tracer, so the player's own body
stayed in vanilla lighting while the world around it was path-traced. This
change brings that state into RT, in three parts.

First, a mod-agnostic extension point: the mainline gains a provider registry
and a per-frame camera-safety declaration, with no mod identity in the core.
The highest integer priority wins; a tie fails closed for the frame with one
WARN, and a false, missing, or throwing safety declaration is treated as
unsafe. The first-person instance's motion identity is -(entityId + 1),
disjoint from the vanilla body's positive id space, so history-map swaps need
no explicit reset. Off by default, the feature short-circuits before the
registry lookup: zero extraction, zero logging when disabled.

Second, the FirstPerson Model bridge. The mod already appends a complete
first-person state during vanilla's extraction pass, so the bridge picks that
state out and hands it to the extension point. No geometry is rebuilt; the
state's own x/y/z already carry the mod's computed offset, so the instance
anchor is correct automatically. The dependency is compileOnly; the mod is
neither bundled nor required at runtime.

Third, the camera entity now uses the first-person state as its single
instance with a fully visible mask when the provider delivers it, falling back
to the baseline mask and anchor otherwise. Dual-instance layouts blacked out
the torso: the offset layout let first-person surface rays hit the co-visible
vanilla body, and co-locating the instances wrapped the torso in the vanilla
head cube, measuring exactly (0,0,0) over 13%-33% of the frame. The
single-instance layout eliminates the black region.

Known issue: parts the provider hides do not participate in shadows, GI, or
reflections, so the player's own shadow has no head. This is the documented
cost of the single-instance layout, accepted in exchange for correct
first-person surface lighting.

Build / test: gradlew build, 52 testcases across 17 testsuites, all green.
Smoke-tested in a Fabric 26.2 instance with FirstPerson Model 2.7.2: the
first-person body participates in RT lighting with no black region.

Dependencies: build.gradle adds a Modrinth maven repository and a compileOnly
maven.modrinth:first-person-model dependency. CI's package job resolves it
from api.modrinth.com on the runner.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>