Emit per-construct diagnostics for unsupported switch forms

[fabiomadge]

Step 2b of PLAN.md §3 — refusal-time diagnostics for switch forms that can't be encoded as if/conditional chains.

Resolves

• Laurel compiler: Switch expression support #401 (switch encoding) — together with Stop UNSUSPEND from restoring switches to JCSwitch nodes #417, this closes the switch-desugaring portion. case null is deferred to Step 9 alongside Option-type synthesis.

Summary

Suspenders' switchToIf/switchExpressionToConditional return Optional.empty() for forms they can't encode. Today those refused trees fall through to LOWER, which mutates them into shapes JavaToLaurelCompiler can't translate, surfacing as a JavaViolationException → method-level skip via Step 1's catch. The error points at the method, not the offending construct.

This PR replaces the fall-through with refusal-time diagnostics emitted at each offending element's source position. The check happens at SUSPEND time, before LOWER runs:

Construct	Diagnostic	Anchor
case A: (statement-form)	notSupported "switch labeled statement group"	case node
case Pattern p when ...	notSupported "case pattern"	case node
case 1 + 1 -> ...	notSupported "non-literal case constant"	label expr
case 0 -> { block } (switch expr only)	notSupported "switch rule block"	body
case 0 -> throw ... (switch expr only)	notSupported "switch rule throw statement"	body

After diagnostics, the offending switch is replaced with a no-op placeholder (empty block for statement form, typed-zero literal for primitive-typed expression form) so LOWER doesn't mutate the refused tree.

Implementation notes

Pre-walk for refusal detection. The encoder silently accepts some refused forms — most notably case 1 + 1, where Attr constant-folds the binary expression and the encoder produces a valid if (selector == 1+1) { ... }. Post-hoc Optional.empty() detection misses these. A pre-walk (hasRefusedConstruct) inspects all labels and body shapes before encoding and is the source of truth for refusal.

Reference-typed switch expressions. Fall through to the existing skip path rather than substituting a typed-null placeholder. Building a typed null is fragile across javac versions, and this path is unreachable today (reference-typed switch expressions trip earlier on type translation).

Reporter wiring. Suspenders gains a Reporter field and a visitTopLevel override matching the pattern used in MethodOrLoopContractCompiler.

Fixture changes

TranslationErrors.java stays @JVerifyTest(skip = "...") — its non-switch markers (lines 18, 24, 125, 148, 155, 160, 183) gate the file flip on Steps 8-9. Two aspirational "switch labeled statement group" markers on multi-label arrow cases (lines 94 and 112) removed: those weren't real refusal targets, multi-label arrow form with a valid body is supported by Step 2a (Switches.switchExprInt exercises case 1, 2 -> 20; and verifies). When the rest of the fixture clears in Steps 8-9, TranslationErrors will flip cleanly.

Base branch

Targets #417 (step-2a/suppress-switch-restoration). Will rebase onto main once #416 → #417 chain merges.

Test plan

• ./gradlew :verifier:test passes — 116 / 0 / 72 (was 115 / 0 / 72 on Step 2a, +SwitchRefusal).
• No new uncaught JavaViolationException stack traces in test output.
• No green → red transitions.
• SwitchRefusal.java covers all five refusal classes end-to-end.