refactor(builtin): <+>-simplify Iter and Map Show and small write sequences

[bobzhang]

Part of the <+ template-writing simplification, split out of the closed
#4195 for reviewability.

• Show for Iter and Show for Map write their delimiters/separators
  through <+ templates with \{cb => ...} writer holes, keeping
  write_object dispatch monomorphic
• base64 == tail and a FixedArray iterator-test builder collapse into
  single <+ writes

Verified: moon check 0 warnings/0 errors, builtin suite 2988/2988.

Generated with SeekMoon

[Copilot]

🟢 Approval recommended

The changes are localized refactors that preserve output semantics, and the updated patterns align with existing <+ usage in builtin without introducing new logic paths.

Pull request overview

This PR continues the ongoing <+ template-writing refactor in builtin by replacing small write_string / write_char sequences with <+ writes, including inline-writer holes (\{cb => ...}) where needed to keep write_object dispatch monomorphic.

Changes:

• Refactors Show for Iter and Map to emit delimiters/separators via <+ templates (using writer holes for write_object).
• Simplifies base64 padding output by writing "==" in a single <+ call.
• Simplifies a FixedArray iterator test’s expected-string builder to a single <+ write per element.

File summaries

File	Description
builtin/linked_hash_map.mbt	Uses <+ templates (with writer holes) for Map[K,V] Show::output punctuation and key/value emission.
builtin/iterator.mbt	Uses <+ templates for Iter[X] Show::output punctuation and separator+element emission.
builtin/fixedarray.mbt	Collapses StringBuilder writes in the iter test to a single <+ "\{x}\n" call.
builtin/console.mbt	Writes base64 "==" padding via a single <+ call.

Review details

• Files reviewed: 4/4 changed files
• Comments generated: 0
• Review effort level: Lite

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[coveralls]

Coverage Report for CI Build 6525

Coverage increased (+0.007%) to 89.2%

Details

• Coverage increased (+0.007%) from the base build.
• Patch coverage: 2 uncovered changes across 1 file (3 of 5 lines covered, 60.0%).
• No coverage regressions found.

Uncovered Changes

File	Changed	Covered	%
builtin/iterator.mbt	2	0	0.0%
Total (2 files)	5	3	60.0%

Coverage Regressions

No coverage regressions found.

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Coverage Stats

Relevant Lines:	18204
Covered Lines:	16238
Line Coverage:	89.2%
Coverage Strength:	274532.43 hits per line

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💛 - Coveralls