refactor(codemode): make the host boundary JSON.stringify plus a short table

[rekram1-node]

The host boundary is JSON.stringify, plus a short table for values that cannot be JSON but whose intent is clear.

program ──► tool args / result    JSON.stringify, then the table below
tool result ──► program           JSON.parse(JSON.stringify(result))

One walker replaces four conversion modes and a second copy of itself.

 data.ts
-copy(value, label, mode: "program" | "data" | "json" | "result", depth, seen, protos?, boundary)
-toProgram / fromData / toData
+toJson(ctx, value, replacer?)     the JSON.stringify walk: toJSON honored, cycles → TypeError
+toBoundary(ctx, value)            toJson plus the table
+fromJson(ctx, json)               host JSON → Obj / Arr

 stdlib/json.ts
-second stringify walker for function replacers
+JSON.stringify(toJson(ctx, value, replacer), properties, indent)

tool-runtime.ts now deals only in Json: arguments arrive converted and results leave as JSON.parse(JSON.stringify(decoded)), so program values never enter it. The conversion sits in one place, Interpreter.tool, which both tools.* calls and search go through. ToolRuntimeError moves there from data.ts.

The table

value at the boundary	before	after
un-awaited promise	rejected with hint	awaited; a rejection fails the program with its reason
Set	array	array
URLSearchParams	query string	query string
Error	{ name, message, ...own }	same (also in-program JSON.stringify)
Uint8Array	rejected with encode hint	same
own __proto__ key	dropped	dropped
RegExp	"/src/flags"	{} like JS
extension handle, generator	rejected with hint	{} like JS
function	rejected	dropped like JS
undefined property in the result	null	dropped like JS
cyclic value	InvalidDataValue	TypeError: Converting circular structure to JSON.
user-defined toJSON	honored only with a function replacer	honored everywhere
tool output	decode → host value → lossy copy	decode → JSON.stringify → JSON.parse

Map stays {} for now; a smarter form is a follow-up.

In-program JSON.stringify keeps JS behavior except for the Error form and a promise, which is a TypeError with an await hint — it cannot await synchronously, and a silent {} there is the footgun the table exists to prevent.

Also gone: the toProgram(...) calls on literals, operator results, and template interpolations in interpreter.ts — those values are already primitives. BigInt literals now fail with a plain TypeError at the literal.

Files

src/data.ts                     239 → 120 lines
src/stdlib/json.ts              second walker deleted
src/tool-runtime.ts             Json in, Json out; ToolRuntimeError lives here
src/interpreter/interpreter.ts  Interpreter.tool; toProgram calls removed
src/interpreter/errors.ts       thrown-value message via console formatter
src/stdlib/{string,object,value,url,console}.ts   toProgram / toData callers
interpreter-support.md          boundary rule rewritten as one paragraph

Checks

bun run check clean; bun test 5125 pass / 0 fail; test262 report unchanged at 3858 pass / 426 known-gap.