fix(extraction): convert remaining fixed-size stacks in extract_channels.c to growable TSNodeStack

[jjoos]

Problem

PR #217 converted most AST traversal functions to use the new growable TSNodeStack (arena-allocated, no hard cap), but extract_channels.c still had 8 functions using fixed-size TSNode stack[CHAN_STACK_CAP] arrays:

• scan_string_consts_python
• extract_channels_python
• extract_channels_go
• extract_channels_java
• extract_channels_csharp
• extract_channels_ruby
• extract_channels_elixir
• extract_channels_rust

When indexing large repositories with deep AST nesting (e.g. 4096+ nodes on a single DFS path), these arrays overflow silently — the capacity guard && top < CHAN_STACK_CAP drops children without warning, and when the stack itself overflows it corrupts adjacent heap memory. The corruption surfaces as a segmentation fault in a later pass (observed in lsp_cross and semantic_edges passes on large repos).

Fix

Apply the same mechanical conversion used in PR #217 to the 8 remaining functions:

Before	After
TSNode stack[CHAN_STACK_CAP]; int top = 0;	TSNodeStack stack; ts_nstack_init(&stack, ctx->arena, CHAN_STACK_CAP);
stack[top++] = ctx->root	ts_nstack_push(&stack, ctx->arena, ctx->root)
while (top > 0)	while (stack.count > 0)
TSNode node = stack[--top]	TSNode node = ts_nstack_pop(&stack)
&& top < CHAN_STACK_CAP capacity guard	removed (arena grows automatically)
stack[top++] = ts_node_child(...)	ts_nstack_push(&stack, ctx->arena, ts_node_child(...))

Testing

Verified by indexing large repositories (1290 files, 13,735 nodes; 3482+ functions) — previously these repos caused segfaults, with this fix all repos index cleanly.