feat: support signed by iteration steps

[tiye]

Summary

• implement signed non-zero finite by steps for arrays and Unicode strings
• preserve actual indices for reverse iteration and reject map by
• add RFC 0100, tests, syntax/manual updates, and regenerated manuals

Validation

• make check
• make docs
• git diff --check

Part of the RFC-driven PR chain; do not merge automatically.

[Copilot]

Pull request overview

This PR extends QuickCoffee’s for … by iteration semantics to support signed, non-zero finite integer steps for arrays and Unicode-scalar string iteration, aligning implementation, tests, RFCs, and regenerated manuals.

Changes:

• VM: store by step as signed i64, start reverse iteration from the last element/scalar, and advance position safely without wraparound.
• Tests: add coverage for negative by steps and “actual index” behavior in reverse iteration for arrays and strings.
• Spec/docs: add RFC 0100 and update related RFCs, syntax docs, and executable manuals (plus regenerated HTML outputs).

Reviewed changes

Copilot reviewed 25 out of 25 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
tests/rfc_core.rs	Adds assertions for negative by steps over arrays and Unicode strings; updates test naming to reflect signed steps.
src/vm.rs	Implements signed-step enumerable iteration and position advancement for arrays/strings.
RFCs/0100-signed-by-iteration.md	New RFC defining signed by iteration contract and acceptance criteria.
RFCs/0095-string-iteration-by-step.md	Updates string-step RFC text to allow non-zero signed steps and references RFC 0100.
RFCs/0070-string-iteration.md	Updates core string iteration RFC to describe signed, non-zero step semantics.
RFCs/0029-stepped-array-iteration.md	Updates stepped array iteration RFC to include negative steps and VM behavior notes.
RFCs/0001-language-core.md	Updates language core description to reflect signed non-zero by for arrays/strings.
RFCs/0000-project-scope.md	Extends implemented RFC range through RFC 0100.
README.md	Updates RFC coverage link to RFC 0100 as the latest.
manuals/manual.zh-CN.qc	Updates executable manual text to describe signed non-zero by steps.
manuals/manual.latin.qc	Updates executable manual text to describe signed non-zero by steps.
manuals/manual.en.qc	Updates executable manual text to describe signed non-zero by steps.
manuals/manual.devanagari-sa.qc	Updates executable manual text to describe signed non-zero by steps.
manuals/manual.classical-zh.qc	Updates executable manual text to describe signed non-zero by steps.
docs/syntax.zh-CN.md	Updates syntax documentation for signed non-zero by iteration behavior.
docs/syntax.en.md	Updates syntax documentation for signed non-zero by iteration behavior.
docs/manual.zh-CN.md	Updates rendered manual markdown to describe signed non-zero by steps.
docs/manual.zh-CN.html	Regenerated HTML manual reflecting updated signed-step docs.
docs/manual.latin.html	Regenerated HTML manual reflecting updated signed-step docs.
docs/manual.en.md	Updates rendered English manual markdown for signed-step iteration.
docs/manual.en.html	Regenerated HTML manual reflecting updated signed-step docs.
docs/manual.devanagari.sa.md	Updates rendered Devanagari manual markdown for signed-step iteration.
docs/manual.devanagari-sa.html	Regenerated HTML manual reflecting updated signed-step docs.
docs/manual.classical-zh.md	Updates rendered classical Chinese manual markdown for signed-step iteration.
docs/manual.classical-zh.html	Regenerated HTML manual reflecting updated signed-step docs.

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