json(vm): fix birth dispatch; unify constructor naming (Box.birth/N); JsonNode factories return JsonNodeInstance; quick: enable heavy JSON with probe; builder: NYASH_BUILDER_DEBUG_LIMIT guard; json_query_min(core) harness; docs/tasks updated

docs/guides/dev-mode.md

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+Dev Mode and Defaults (Plan)
+
+Overview
+- Goal: make Nyash simple to run like other languages. Defaults should be obvious; experiments should be opt‑in.
+- Two modes:
+  - Production (default): quiet, stable; only stable features on.
+  - Development (`--dev`): safe dev defaults on; experiments enabled in observe mode; helpful diagnostics.
+
+Current
+- Use `--dev` to enable development defaults:
+  - `nyash --dev script.nyash`
+  - Enables AST using + Operator Boxes (observe) by default. Output remains stable.
+- Dev shortcuts remain available:
+  - `./tools/opbox-json.sh` – JSON Roundtrip/Nested with Operator Boxes
+  - `./tools/opbox-quick.sh` – Quick suite with Operator Boxes
+- Using guard:
+  - Duplicate `using` (same file imported twice or alias rebound) is a hard error with file:line hints.
+  - Fix by removing/consolidating duplicates.
+
+Defaults mapping
+- Production (default)
+  - using: ON (SSOT). AST merge only when nyash.toml defines the module (no implicit).
+  - Operator Boxes: OFF (no behavior change from legacy).
+  - Traces: OFF.
+- Development (`--dev`)
+  - using: AST merge ON (SSOT + AST prelude enabled by default).
+  - Operator Boxes: observe ON (stringify/compare/add calls visible; results not adopted → output is stable).
+  - Traces: OFF by default; can enable selectively via `NYASH_TRACE` (to be introduced) or legacy flags.
+  - Duplicate `using`: error (with line numbers).
+
+Flag consolidation (mid‑term)
+- `NYASH_OPERATOR_BOXES=all|none|comma-list` → expands to legacy flags.
+- `NYASH_TRACE=all|vm,box,print,if,loop` → expands to legacy trace flags.
+- Old flags remain; new keys take precedence; final resolved state is logged when `NYASH_CLI_VERBOSE=1`.
+
+nyash.toml profiles (long‑term)
+- Example:
+  [profiles.json-dev]
+  operator_boxes = ["stringify", "compare", "add"]
+  using_ast = true
+  trace = []
+
+  [profiles.debug]
+  operator_boxes = "all"
+  trace = ["vm", "box", "if", "loop"]
+  verbose = true
+
+Priority
+- CLI `--profile` > explicit env > nyash.toml defaults > built‑in defaults.
+
+Acceptance
+- `nyash script.nyash` runs with stable defaults.
+- `nyash --dev script.nyash` enables AST using + Operator Boxes (observe) and passes JSON Roundtrip/Nested.
+- Smokes use `--dev` path when appropriate; profiles remain as a convenience.