fix: resolve Cranelift IR verification errors for extern function signatures

Three issues caused 46 functions to be skipped during compilation:

1. Return type mismatches: Return values weren't coerced to match the
   function signature's declared types. All three Return terminator
   handlers now use coerce_value, and bare returns (0 values when
   signature expects N) are padded with zero values.

2. Cross-size int/float coercion: coerce_value only handled same-size
   bitcasts. Added fcvt_from_sint for int->float and fcvt_to_sint_sat
   for float->int with different bit widths.

3. Extern 0-param placeholders: Import resolution created HirFunctions
   with empty param lists for ZRTL symbols. Added prescan_extern_call_sites
   to infer correct param types from call sites before declaration, and
   fall back to ZRTL plugin signatures when available. Duplicate symbol
   declarations (e.g. tensor_sqrt and $Tensor$sqrt) reuse existing FuncIds.

Reduces CRANELIFT WARN from 46 to 4 (remaining are legitimate ZRTL API
mismatches needing List-unpacking wrappers in tensor.zynml).

Author: darmie