Add runtime support for the three string operations specified in
Chapter 4, Section 4.13 of the language spec. These are the first
dynamic string operations in the compiler — prior to this, only
string constants were supported.

Implementation:
- Register string_length, string_concat, string_slice as built-in
  functions in the type checker environment
- Add WASM codegen for all three operations using the existing bump
  allocator ($alloc) and byte-copy loops for string_concat/string_slice
- Add return type inference for string operations in both WASM and
  Vera type inference paths
- Add string built-ins to the known-functions whitelist in the
  cross-module call scanner

Tests: 18 new tests (5 checker + 13 codegen), all passing.
Example: examples/string_ops.vera demonstrates all three operations.

Partial progress on aallan#52 and aallan#134.

Co-Authored-By: Claude <noreply@anthropic.invalid>

…uilt-ins

Add five more string/conversion built-in functions to complete the set
needed for string-processing Vera programs:

- char_code(String, Int -> Nat): returns byte value at given index
- parse_nat(String -> Nat): parses decimal string to natural number,
  skipping leading spaces
- parse_float64(String -> Float64): parses decimal string with optional
  sign and decimal point to 64-bit float
- to_string(Int -> String): converts integer to decimal string
  representation (handles negatives)
- strip(String -> String): trims leading/trailing ASCII whitespace
  (space, tab, CR, LF); returns a view without allocation

Implementation details:
- parse_float64 handles sign, integer part, and fractional part using
  f64 arithmetic in WASM
- to_string uses a 20-byte temp buffer with reverse digit extraction
- strip returns a pointer into the original string (zero-copy)

Tests: 34 new tests (5 checker + 29 codegen), all passing.
Updated string_ops.vera example to demonstrate all eight string built-ins.

Further progress on aallan#52 and aallan#134.

Co-Authored-By: Claude <noreply@anthropic.invalid>

- string_length: return type INT → NAT (spec §4.13 says non-negative)
- string_slice: param types INT, INT → NAT, NAT (spec §4.13 says
  non-negative positions)
- Update test_string_slice_ok to use @nat params

Co-Authored-By: Claude <noreply@anthropic.invalid>