feat(solo/stdlib): json_parse / json_stringify (+ fix string-literal escapes)

[hyperpolymath]

Second Phase-2 stdlib primitive (tracker #45), unblocked by Map (#46/#49).

What

• json_parse(s) -> Value / json_stringify(v) -> String, backed by serde_json (promoted to a non-optional dep of my-lang; serde derive stays behind the serde feature).
• Representation pairs with the Map builtins: object↔Record, array↔Array, integral number→Int else Float, string↔String, bool↔Bool, null↔Unit. Non-serializable values (functions/AI results, NaN/Inf floats) error cleanly.
• Object keys serialize sorted (serde_json default BTreeMap) — same deterministic ordering as map_keys.

Prerequisite bug fixed (in scope — feature is unusable without it)

The lexer stored the raw slice between quotes and escape sequences were decoded nowhere. "a\"b" yielded the literal 4 chars a \ " b, and json_parse("{\"k\":1}") — issue #45's own repro — could not parse. parse_string_literal now decodes \" \\ \/ \n \t \r \0 and \uXXXX / \u{...}. Unknown escapes pass through leniently (backslash dropped, char kept) so no previously-valid program changes behavior except where it was already wrong.

Validation

• examples/json.my runs and matches its documented output.
• New regression tests: test_eval_json_roundtrip (sorted-key round-trip) and test_eval_string_escapes. Full suite 17 passed, 0 failed.
• New code is rustfmt-clean and clippy-clean. Pre-existing fmt drift elsewhere in stdlib.rs left untouched (out of scope, consistent with PR feat(solo/stdlib): string-keyed Map/dict builtins #49).

Scope notes

vscode-extension/package-lock.json (pre-existing untracked) not included. Cargo.lock is gitignored in this repo.

Refs #45
Refs #47

🤖 Generated with Claude Code

[github-actions]

🔍 Hypatia Security Scan

Findings: 35 issues detected

Severity	Count
🔴 Critical	6
🟠 High	11
🟡 Medium	18

⚠️ Action Required: Critical security issues found!

View findings

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