feat(stdlib): add @acton/json decoder module

[zakrad]

Summary

Adds a small, read-only JSON decoder to the Acton stdlib as @acton/json. It is
the natural companion to fs.readFile, which can load a JSON file as a string
but offers no way to parse it.

Implements the proposal in #1096.

API

Values are addressed with a JSON Pointer (RFC 6901), e.g. /token/decimals or
/items/0:

fun json.getInt(src: string, path: string): int?
fun json.getString(src: string, path: string): string?
fun json.getBool(src: string, path: string): bool?
fun json.exists(src: string, path: string): bool

val src = fs.readFile("fixtures/jetton-metadata.json");
if (src != null) {
    val decimals = json.getInt(src!, "/decimals") ?? 9;
    val name = json.getString(src!, "/name") ?? "unknown";
    val mintable = json.getBool(src!, "/mintable") ?? false;
}

Design

• Lookup semantics mirror @acton/env: invalid JSON, a missing path, or a
  type mismatch all return null, so ?? can supply a default.
• JSON Pointer paths via serde_json's built-in Value::pointer() — no
  custom path parser.
• getInt accepts a JSON integer, or a JSON string holding a decimal/0x-hex
  integer. Floats return null. Bare numbers beyond 64 bits are not reliably
  representable, so they return null rather than a lossy value; encode such
  values as JSON strings to carry full 257-bit integers losslessly.
• getBool distinguishes a present false (returned as false) from an
  absent/non-boolean value (null).

Implementation

• Host handlers in src/ffi/json.rs (EXTCALL ids 300–303), registered in
  src/ffi/mod.rs, following the existing env/fs pattern.
• Tolk surface in lib/json.tolk.
• Reuses serde_json, already a dependency; no new crates.

Testing

• New integration tests in
  tests/integration/test_runner/json_reads_typed_values_by_pointer_tests.rs
  cover numbers/strings/hex/floats, nested paths, array indices, present-false
  vs null, missing paths, and invalid JSON.
• Verified locally: just fmt-check, cargo clippy --workspace --all-targets --all-features -- -D warnings, typos, acton docgen --check, acton fmt --check, acton check, and acton test (native stdlib suite).

Docs

Generated docs/content/docs/standard_library/json.mdx and the overview index
via acton docgen; docgen --check passes.

Closes #1096

Note

Prepared with the help of an AI coding assistant; reviewed and locally verified by the author before submission.

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