M10: spatial performance fields (pan/reverb/glide)

[jamiesun]

What

Implements the on-hold roadmap item spatial performance fields — the "near→far→wide→pull back" spatial narrative from real film-scoring work is now expressible in the DSL. Bumps version to 0.2.0.

Three optional per-track fields, all with strictly deterministic MIDI semantics:

Field	Range	Compiles to
pan	0.0..=1.0	CC10 at tick 0 (round(v*127))
reverb	0.0..=1.0	CC91 at tick 0
glide	0.0..=1.0, melody-only	tail portamento via pitch bend, clamped ±2 semitones, PPQ/8 ramp grid

Design notes

• Glide is computed after performance transforms (reads post-swing/humanize onsets); the ramp window is capped at the next onset so legato overlap glides into the new note; loop scenes glide the last note toward the first with the reset on the loop boundary — seam continuous by construction.
• Pitch-bend events rank between note-offs and note-ons at the same tick, so bend-free scenes stay byte-identical (golden forest.mid untouched).
• glide on a non-melody track and out-of-range values are validation errors with field paths.
• Backends: FluidSynth/TiMidity honor all three; sfizz honors pitch bend (±200-cent default matches the clamp), CC10/91 only if the .sfz maps them — same graceful degradation as articulation.

Verification

• 13 unit + 72 E2E tests green; fmt + clippy (-D warnings) clean.
• 3 new E2E tests (acceptance-matrix rule 5): spatial_fields_emit_cc_and_pitch_bend_deterministically, spatial_build_keeps_loop_sample_exact, validate_rejects_bad_pan_and_glide_on_non_melody.
• Real-render smoke test: pan: 0.1 → left/right RMS 2073/515 vs centered 1461/1512 without the fields.
• Docs: roadmap M10 section + capability list + acceptance-matrix row; skill reference field table + backend note.