state_exec: parser.max_depth 100000; the last recursive walks over parsed data

[transfix]

Finishes what #657 and #658 left over parsed data. Nothing in libcvc walks parsed data on the native stack any more, so parser.max_depth stops being a stack guard and its default rises from 256 to 100000 (the user's choice). 100000 is the depth every walker is tested at on a 64 KiB stack.

What changes

• Ariadne channel lint. collect_channel_refs (loader.cpp) keeps its open lists on a heap stack. The old cutoff at 256 levels is gone, so a msg-* reference at any depth is checked.
• stream-capture-audio graphs. A built tone/gain/mix graph pulls each chunk through one native call per level on the producer thread, and frees itself the same way. A builder loop would not help, so depth is capped instead: a SPEC nesting mix/gain more than 64 levels deep is refused with an error ("... more than 64 levels deep") instead of built.
• state-set of a list or dict prints it as a data job (run_data_job(make_print_job(...))). With data.step_nodes > 0 the printing runs a slice per step like str, and the node is written once the whole string is ready. Scalars are unchanged.
• Test-only tree-walking evaluators (evaluator, async_evaluator). Their macro substitute runs the stackless evaluator's walk whole instead of recursing per template level.
• parser.max_depth defaults to 100000. The header comment, the grammar doc ("Nesting", "Parser settings", the P1b row) and the pycvc comment/README say why. Values crossing into Python go through pycvc's converter, which has its own depth limit (pycvc: state_exec parity part 1 (values, parsing, parse_error, value codec) #661).

Not in this PR

PR B, on top of #665's call_function:

• a cap on native→native nesting (apply/send on a self-containing list, class constructor/init cycles);
• class teardown without a native frame per superclass;
• generators and defclass methods without nested evaluators;
• the snapshot codec walking closures and environments iteratively.

The pycvc converter (value_to_py/py_to_value) is left to the parity stack (#661 and its follow-up), which replaces it.

Tests

• StateExecDeepData.ParsedDataAtTheDefaultDepthLimit: on the small stack, a script at the default limit parses, compares, prints and frees, and one level more is a parse_error. It passes at 256 KiB and 64 KiB (CVC_DEEP_DATA_STACK_KIB=64).
• DataStepsScheduler.StateSetOfADeepValueSlices: a 300-deep list takes data steps, the node is empty mid-walk, and it ends holding the printed list.
• AriadneChannelLint.SeesChannelsNestedAnyDepth: an undeclared channel 1000 levels down fails a strict load.
• cvcgl_media_audio: a 64-level gain chain is built, and a 65-level one is refused with the cap's error.
• The default's asserts (parser, grammar P1b row, integration settings tests, pycvc) now say 100000. NestingLimitStopsRunawayInputBeforeTheStackDoes feeds a million ( and checks the error is at column 100001.

Local results (MSVC 14.38 Release, Windows)

• On the merge with master (state_exec: the stdlib materializers run as sliced data steps, and predicates get them #665/state_exec: max_memory covers the measured heap; collect reference cycles #667 in): all 29 state_exec, Ariadne, state, state_list and app suites pass (1680 tests); state_exec_deep_data_test also at 64 KiB; pycvc test_pycvc_exec.py and test_pycvc_ari.py 23 passed.
• Before the merge: all 27 state_exec, Ariadne, state and app suites passed.
• cvcgl_media_audio passed in a cvcGL build (CVC_BUILD_CVCGL=ON, SDL and imgui on).
• clang-format 18 is clean on the changed lines.