test: characterize request coalescing

[Fyko]

what

• add a database-free harness for request coalescing
• characterize leader/follower fan-out, cancellation, query-type isolation, and empty results
• capture six known runtime failures and two query-identity failures as ignored plan 003 tests
• test generated read-query identity behavior at the macro and example layers

why

request coalescing is scyllax's defining behavior, but its runtime and generated identity semantics had no direct regression coverage. this establishes the safety net needed before changing the runtime or upgrading the scylla driver.

verification

• focused coalescing suite passed 10 consecutive runs
• workspace tests passed with all features
• docs built successfully
• formatting check passed
• every ignored regression failed independently for its intended reason