[dsl] Silently dropped inputs need diagnostics: on directives, modifier arguments, gap_direction coercion, lateral anchoring

[JArmandoAnaya]

What

Several DSL inputs are dropped silently — no diagnostic at all — violating the coverage matrix's own contract that everything unsupported is "reported by name, never silently dropped":

1. on directives — lower.cpp visits only ModifierApplication and Behavior members (:1534-1555); DoMemberKind has no On case (:1143-1153). Matrix claims §7.6.2.5 on is Exec In; reality: dropped without a word. (Related: the §7.6 table rows for until/wait @event/emit/call say In while the code reports them — matrix side tracked in the docs truth-pass issue.)
2. Modifier arguments ignored on all 17 modifiers — movement_mode, track, shape are declared on every §8.9 modifier (stdlib.cpp:1244-1394) but only at is ever read (lower.cpp:707, anchor_of). speed(direction:) (§8.9.4) and position(project_on_route:) (§8.9.2) likewise ignored. The matrix says "Post (default profiles only — documented)" but the promised structured diagnostic does not exist.
3. lateral(at: end) inconsistency — lower.cpp:904 bypasses anchored_dynamics, so the phase-length check every other anchored modifier applies (warning at :650) never fires for lateral.
4. No tests exercise the existing until/emit/call/non-default-overlap warnings (grep of frontends/dsl/tests finds parse-only hits) — the "unsupported paths diagnose, never crash" criterion is untested for the P8 constructs.

Done when

• Every ignored input emits a §-cited UnsupportedFeature (or is implemented); on directives specifically stop vanishing
• lateral goes through the same anchoring path as the other modifiers
• Diagnostic-assertion tests for on/until/emit/call/overlap/modifier-argument paths