Parametrize the left-to-right handoff scene (cameras, FOV, path, frames)

[bamdadd]

build_handoff_ltr_scene() currently hard-codes camera positions, FOVs, the path, and frame count. Parametrize it so the scene is a family, not one fixture — useful for sweeps (how much overlap? how long a blind gap? how many cameras?).

Scope

• Add optional keyword args (with the current values as defaults so the golden/byte-stable behaviour is unchanged when called with no args): number of near cameras, camera FOV, line length / start-end x, num_frames, and the wide camera's placement.
• Keep determinism (no unseeded RNG). Defaults must reproduce the exact current manifest.

Acceptance

• build_handoff_ltr_scene() with no args is byte-identical to today (add/extend a determinism test).
• A parametrized variant (e.g. more cameras, wider overlap) builds a valid manifest and its coverage differs as expected (a test).
• Typed, ruff + mypy clean.

Pointers

• src/multicam_sim/handoff_ltr.py, tests/test_handoff_ltr.py, src/multicam_sim/mtmc.py (constants-at-top style).

[Nitjsefnie]

Heads-up before anyone picks this up: build_handoff_ltr_scene() and src/multicam_sim/handoff_ltr.py are not on main — they only exist in #93 and #94, which you closed. So the acceptance criterion here ("byte-identical to today with no args") has no baseline to compare against, and the same premise affects #88, #89, #90 and #92.

Leaving these alone rather than guessing at a scope. Happy to take any of them once the underlying scenes are on main — #65 was unaffected, so I have been working that one instead.