Compile functions with $-prefixed parameters instead of bailing

[revarbat]

Summary

• $-prefixed parameters (e.g. function f($fn) = ...) used to bail compilation of the whole containing declaration; they now compile, binding through ctx.dyn instead of a slot since every reference already compiled to Op::LoadDyn regardless.
• Same fix also unblocks a FunctionLiteral closure with its own $-param, since compileFunctionLike is shared between plain functions and closures.
• Positional binding, named binding, and defaults (including the "declared-but-unbound shadows to undef" rule) all route to ctx.dyn consistently for $-params, mirroring the interpreter's existing bindCallArgsInto/applyDefaults behavior.
• No tail-call-specific handling was needed: every trampoline hop rebinds arguments through the same helper a genuine call uses, and $-var dynamic scoping already threads correctly across hops.

Test plan

• Rewrote the two tests that asserted the old bail behavior (DollarPrefixedParameterCompiles, ClosureWithDollarParameterNowCompilesToo) to instead prove compilation via the file's existing fast-continue stop-count technique (one of the old assertions turned out to be unreliable regardless of the fix -- nullopt fast-continue forces interpretation for every function, not just the one under test).
• Added DollarParameterThreadsThroughCompiledTailRecursion -- a $fn parameter threaded through 50,000 compiled tail-recursive hops.
• Full suite: 681/681 passing.
• Manual smoke tests (compiled vs interpreted agreement): declared-with-default, positional binding, named override, declared-but-unbound-shadows-to-undef, and deep tail recursion.

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