feat(format): REPL pretty-printing + distinct list/vector brackets

[singaraiona]

Problem

In the REPL, a generic LIST was indistinguishable from a typed vector — homogeneous-atom lists collapsed to [...], so (list null null) printed [null null]. And dicts/nested structures always rendered on a single line, hard to read once nested.

Changes (REPL display / mode 1 only)

1. Distinct list vs vector brackets. A generic list now renders with parens, distinct from a vector:

(list null null) -> (null null)     ;; was [null null]
(list 1 2 3)     -> (1 2 3)
[1 2 3]          -> [1 2 3]

The homogeneous-atom → [..] collapse is kept for mode 0 (round-trippable), where fmt_eq and the whole suite rely on list/vector display equivalence. (Removing it for all modes broke 48 tests; gating to mode 1 is the correct scope.)

2. Width-triggered pretty-printing (ray_fmt_pp / ray_fmt_pp_print, wired into repl_print_result). Dicts/lists render on one line if they fit the row width (80); otherwise they break JSON-style with 2-space indent, and each nested child stays compact if it fits:

{
  name: "alice anderson"
  age: 30
  tags: ("engineer" "climber" "baker")
  address: {city:"springfield" zipcode:12345 country:"wonderland"}
}

Breaking is recursive; the broken form caps at FMT_PP_MAX_ROWS (20) then ...; vectors stay single-line (truncated at width). REPL-only — ray_fmt, mode 0, and table/vector cells keep their current single-line output, so tables and the test suite are unaffected.

Refactor. Extracted fmt_dict_key / fmt_dict_val from fmt_dict so the pretty-printer reuses the per-type key/value synthesis instead of duplicating it.

Tests

Added to test_format.c (TDD): the bracket fix (format/list/homogeneous_parens) and five pretty-printer cases — fits / breaks / nested-stays-compact / list-fits / scalar+vector passthrough. Updated one incidental existing assertion (empty_strs) to the new mode-1 list form.

Full suite passes under the sanitizer build (3447 passed, 0 failed, 3 pre-existing skips); REPL verified manually.

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