Retain transition coverage in exploration

[SandroMaglione]

Summary

• expose exact definition- and branch-level transition coverage on every MachineTest.Exploration
• reuse one retained-transition collector across trace coverage and bounded exploration
• count startup, collapsed/self-loop, simultaneous, and state-limit plans while excluding unplanned depth/transition frontiers

Changeset

• Added or updated for a library or package-metadata change
• Not required because this PR does not change src/ or package.json

Validation

• pnpm check
• Relevant example checks, when examples changed (no examples changed)
• Automated type-performance measurement passed or was not required
• Automated runtime- and memory-performance measurement passed or was not required

Local pnpm perf:types and pnpm perf:runtime checks passed; CI comparisons remain authoritative.

[github-actions]

Type performance

Measured with TypeScript 6.0.3 and skipLibCheck=true.

Scenario	Base	PR	Difference
Effect only	55	55	0 (0.0%)
Import effect-machine	55	55	0 (0.0%)
Machine.defineStates (3 states)	3,039	3,039	0 (0.0%)
Machine.make (3 states, 2 events)	10,439	10,439	0 (0.0%)
machine.handle (3 states, 2 transitions)	53,206	53,206	0 (0.0%)
Machine.transition (10 conditional cases)	116,110	116,110	0 (0.0%)
Machine.invoke (state-dependent Effect)	57,187	57,187	0 (0.0%)
machine.handle (depth 24)	188,289	188,289	0 (0.0%)
machine.handle (wide depth 16)	219,355	219,355	0 (0.0%)
machine.handle (parallel/history/choice)	132,085	132,085	0 (0.0%)
machine.handle (4 successive calls)	137,762	137,762	0 (0.0%)
machine exact input/output/error/services	114,832	114,832	0 (0.0%)
execution adapter readiness	124,664	124,664	0 (0.0%)

Marginal instantiations are measured against the matching setup without that API call:

Scenario	Base	PR	Difference
Import effect-machine	0	0	0
Machine.defineStates (3 states)	2,984	2,984	0 (0.0%)
Machine.make (3 states, 2 events)	7,392	7,392	0 (0.0%)
machine.handle (3 states, 2 transitions)	42,767	42,767	0 (0.0%)
Machine.transition (10 conditional cases)	106,181	106,181	0 (0.0%)
Machine.invoke (state-dependent Effect)	48,341	48,341	0 (0.0%)
machine.handle (depth 24)	168,599	168,599	0 (0.0%)
machine.handle (wide depth 16)	200,664	200,664	0 (0.0%)
machine.handle (parallel/history/choice)	112,409	112,409	0 (0.0%)
machine.handle (4 successive calls)	122,216	122,216	0 (0.0%)
machine exact input/output/error/services	100,741	100,741	0 (0.0%)
execution adapter readiness	62,727	62,727	0 (0.0%)

Check times (informational)

Scenario	Base	PR
Effect only	0.03s	0.03s
Import effect-machine	0.03s	0.03s
Machine.defineStates (3 states)	0.09s	0.10s
Machine.make (3 states, 2 events)	0.16s	0.16s
machine.handle (3 states, 2 transitions)	0.31s	0.31s
Machine.transition (10 conditional cases)	0.51s	0.51s
Machine.invoke (state-dependent Effect)	0.36s	0.38s
machine.handle (depth 24)	0.68s	0.70s
machine.handle (wide depth 16)	0.73s	0.72s
machine.handle (parallel/history/choice)	0.56s	0.57s
machine.handle (4 successive calls)	0.58s	0.57s
machine exact input/output/error/services	0.52s	0.53s
execution adapter readiness	0.52s	0.51s

Type instantiations are the comparison metric. Check time varies with runner load and is informational only.