fix: add cycle detection to TOML renderer (manifestTomlEx)

[He-Pin]

Motivation

std.manifestTomlEx has its own recursive traversal in renderTableInternal that bypassed the Materializer's IdentityHashMap-based cycle detection. Cyclic objects like {a: $} caused OutOfMemoryError instead of the clean "Stackoverflow while materializing" error that JSON and YAML renderers produce.

Modification

• Add depth: Int and visited: IdentityHashMap[Val.Obj, Boolean] parameters to renderTableInternal, matching the pattern in Materializer.MaterializeContext
• Check depth against maxMaterializeDepth and detect revisited objects at method entry; remove from visited set in finally block (backtracking)
• Wrap evalRhs in try-catch for StackOverflowError/OutOfMemoryError
• Add 3 cyclic reference tests: manifestJsonEx, manifestTomlEx, manifestYamlDoc — all using {a: $} pattern

Result

Cyclic objects now produce "Stackoverflow while materializing" error instead of OOM, consistent with JSON and YAML renderers. All existing TOML tests (array, null, standard) continue to pass.

Cross-implementation comparison

Test case: std.manifestTomlEx({a: $}, " ") (cyclic self-reference)

Implementation	Result	Detection mechanism
go-jsonnet v0.22.0	✅ RUNTIME ERROR: max manifest depth exceeded, possible infinite recursion	Depth limit in TOML renderer
jrsonnet v0.5.0-pre99	✅ stack overflow, try to reduce recursion, or set --max-stack to bigger value	Stack overflow detection
C++ jsonnet HEAD-2ef32ab	❌ HANGS (process killed after 5s timeout)	No cycle detection — same bug as sjsonnet pre-fix
sjsonnet (before)	❌ java.lang.OutOfMemoryError: Java heap space	No cycle detection in TOML renderer
sjsonnet (after)	✅ sjsonnet.Error: Stackoverflow while materializing, possibly due to recursive value	IdentityHashMap + depth limit

Key observations

• go-jsonnet is the only implementation with proper TOML-specific cycle detection (depth limit)
• C++ jsonnet has the same bug as sjsonnet had: TOML renderer lacks cycle detection, causing hang/OOM
• jrsonnet catches stack overflow generically, but still detects the cycle
• sjsonnet (after) now matches go-jsonnet's robustness with IdentityHashMap-based detection

Test plan

• std.manifestTomlEx({a: $}, " ") produces clean error instead of OOM
• std.manifestJsonEx({a: $}, ...) continues to work (already had detection)
• std.manifestYamlDoc({a: $}) continues to work (already had detection)
• All existing TOML tests pass (array, null, standard cases)
• Error message matches JSON/YAML renderer format