feat(input): compose held movement into one semantic direction

[zoeyrose]

Outcome

Implement independent Rust/SDL3 held-direction resolution so active perpendicular movement bindings produce one deterministic eight-way semantic movement intent. Track replacement-client parity separately from the completed Classic work documented by atrinik/classic#62 and atrinik/classic#107; those issues are behavior specifications, not implementation sources.

Scope and invariants

• Extend the semantic direction/action model to represent north, northeast, east, southeast, south, southwest, west, and northwest.
• Resolve active semantic movement bindings after input-context/focus arbitration rather than composing raw keycodes, so rebindings, duplicate bindings, keyboard, and controller input share the same behavior.
• Compose all eight perpendicular pairs: N+E→NE, E+S→SE, S+W→SW, W+N→NW, NW+NE→N, NE+SE→E, SE+SW→S, and SW+NW→W.
• Produce the same result for either press order and for bindings received in the same SDL poll or in successive polls.
• Drive each chord through one logical held/repeat stream; constituent repeats must not alternate directions, double cadence, or leave stale queued movement.
• Recompute immediately on partial release and continue in the remaining direction. Final release/cancel emits the required stop exactly once.
• Clear held ownership on focus/context loss, disconnect, device removal, or rebinding without leaking gameplay input from text fields or focused UI.
• Preserve standalone movement, explicit diagonal movement, and the run/fire/stay semantics owned by Rebuild modern controls and onboarding around semantic actions #14. Opposite pairs and combinations of three or more directions need deterministic, non-stuck behavior but no new composition rule.
• Keep resolution in a pure, bounded client layer; map the resolved intent through released Game Protocol 1 adapters without depending on Classic code or packet behavior.

Acceptance criteria

• All eight pairs resolve correctly for both press orders and same-poll/sequential input.
• A held chord emits one direction at normal repeat cadence with no constituent glitch or doubled rate.
• Partial release resumes the remaining direction immediately; final release stops exactly once.
• Tests cover custom and duplicate bindings, keyboard/controller parity, run/fire/stay interaction, focus/context transitions, device removal, rebinding, queue pressure, and reconnect cleanup.
• Opposite and three-or-more-direction input is deterministic and cannot stick movement.
• Pure resolver tests use fake input/time and require no display, GPU, network, server, or ambient user state; supported SDL3 integration paths also pass.
• The behavior-parity matrix in Complete the clean-room client behavior-parity matrix #37 records the result, and Rebuild modern controls and onboarding around semantic actions #14 consumes the resolver through its shared action catalog.
• tools/validate.sh and git diff --check pass.

Dependencies and coordination

Coordinate with #14 for the action catalog/input contexts and #12 for the released Game Protocol 1 adapter. This focused issue owns direction composition and held-stream lifecycle; #13 owns authoritative movement interpolation after an action is accepted.

Licensing

Implement source and tests independently under MIT from public behavior specifications. Do not copy, translate, or use Classic source/tests as an implementation template.