refactor: use <+ template writing in place of write_string sequences

[bobzhang]

Replaces literal and sequence StringBuilder/&Logger writes with the <+
template writer across Show implementations and streaming helpers, part of
the ongoing project-wide simplification from write_string sequences to
<+ template writing.

Changes:

• builtin: BytesView Show and ToJson now share one
  BytesView::escape_to streaming helper; Char/StringView writes of
  non-printable \u{...} escapes collapse into a single template; Iter
  and Map Show stream through <+; a base64 == tail and a
  FixedArray iterator-test builder simplify.
• diff: Range/HunkHeader Show stream via inline-writer holes.
• hashmap, set: HashMap/Set Show collapse punctuation writes
  into <+ templates.
• json: ParseError and the deprecated Json Show impls collapse
  write sequences into templates.
• json_path: JSON Pointer rendering streams the recursive path through
  one template per step instead of interleaved write_* calls.
• v128: zero-padded hex formatting streams through a new
  u64_hex_to helper used by the Show template.

\{l => ...} inline-writer holes preserve monomorphic write_object
dispatch, so output is unchanged throughout (full test suite: 7499/7499).

This re-applies the still-relevant content of the closed
#4110 (simplify-template-writing) on top of current main; the JSON
stringify/escape_to part of that PR was superseded by the merged
SIMD need_escape/write_escaped work, so it is intentionally not
re-applied.

Generated with SeekMoon

[coveralls]

Coverage Report for CI Build 6523

Coverage decreased (-0.02%) to 89.176%

Details

• Coverage decreased (-0.02%) from the base build.
• Patch coverage: 5 uncovered changes across 2 files (28 of 33 lines covered, 84.85%).
• No coverage regressions found.

Uncovered Changes

File	Changed	Covered	%
set/linked_hash_set.mbt	3	0	0.0%
builtin/iterator.mbt	2	0	0.0%
Total (10 files)	33	28	84.85%

Coverage Regressions

No coverage regressions found.

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Coverage Stats

Relevant Lines:	18163
Covered Lines:	16197
Line Coverage:	89.18%
Coverage Strength:	275405.03 hits per line

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💛 - Coveralls

[Copilot]

🟡 Changes recommended

It introduces a new exported builtin API surface (BytesView::escape_to) and related interface drift that appears inconsistent with the stated intent for internal-only helpers, and should be resolved before approval.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Pull request overview

Refactors a set of Show/streaming helpers to use the <+ template writer (and inline-writer holes where needed) instead of sequences of write_string/write_char calls, aiming to keep output stable while simplifying streaming code.

Changes:

• Introduces/uses small streaming helpers (e.g., u64_hex_to, BytesView::escape_to) to write directly into a Logger.
• Converts several Show implementations (and a few related helpers/tests) to emit punctuation and structured output via <+ templates.
• Simplifies a few formatting/escaping sequences by collapsing them into single templates.

File summaries

File	Description
v128/simd_basic.mbt	Adds u64_hex_to and updates V128 Show to stream via <+ with inline-writer holes.
set/linked_hash_set.mbt	Switches Set Show punctuation writes to <+.
json/types.mbt	Collapses ParseError/deprecated Json Show output into <+ templates.
json/json_path.mbt	Streams JSON Pointer token/path rendering using <+ templates.
hashmap/utils.mbt	Refactors HashMap Show output to a single <+ template per entry.
diff/hunk.mbt	Streams Range and HunkHeader Show output via <+ (with some interpolation changes).
builtin/show.mbt	Collapses non-printable \u{...} escaping writes into a single template.
builtin/pkg.generated.mbti	Updates generated builtin interface (currently includes BytesView::escape_to).
builtin/linked_hash_map.mbt	Switches Map Show punctuation/object writes to <+ templates.
builtin/iterator.mbt	Refactors Iter Show output to stream via <+.
builtin/fixedarray.mbt	Simplifies an iterator test’s output builder using <+ interpolation.
builtin/console.mbt	Collapses base64 padding writes into a single literal <+ "==".
builtin/char.mbt	Collapses non-printable \u{...} escaping writes into a single template.
builtin/bytesview.mbt	Introduces BytesView::escape_to helper and rewrites BytesView Show/ToJson to share it.

Review details

• Files reviewed: 14/14 changed files
• Comments generated: 3
• Review effort level: Lite

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[bobzhang]

Splitting this into reviewable, per-area PRs as requested:

• refactor(builtin): dedupe BytesView escaping and template escape writes #4196 refactor(builtin): dedupe BytesView escaping and template escape writes
• refactor(builtin): <+>-simplify Iter and Map Show and small write sequences #4197 refactor(builtin): <+>-simplify Iter and Map Show and small write sequences
• refactor(json): template the ParseError/Json/JsonPath Show impls #4198 refactor(json): template the ParseError/Json/JsonPath Show impls
• refactor(hashmap, set): remove the deprecated Show impls #4199 refactor(hashmap, set): collapse Show punctuation into <+> templates
• refactor(diff, v128): stream Show output through <+> and writer helpers #4200 refactor(diff, v128): stream Show output through <+> and writer helpers

Each PR is independently verified and CI will run per PR. Closing this
umbrella PR in favor of the five above.