chore: augmented assignment review follow-ups 🧹

[timfennis]

Follow-ups from the review of #192, stacked on #193. The review's bigger findings (fuzzer metadata wiring) were made obsolete by the temp-layout refactor in #193; these are the pieces that survived.

Changes

• analyser: named the Binding::None arms in the augmented-assignment plan match (fixes the match_wildcard_for_single_variants clippy warning introduced by fix: fixed augmented assignment 🦾 #192) and extracted the twice-duplicated "index type is an int-range slice" check into index_type_is_slice.
• compiler: compile_batch now takes the expression iterator directly instead of forcing every caller to .collect() into a Vec first.
• manual: the augmented-assignment page only documented variable targets; added sections for indexed/slice/string targets, the once-each evaluation-order guarantee, and the type-checking rules (in-place operators preserve the target type, fallback operators may widen). All new examples are verified to run.

Remaining clippy warnings (chunk.rs use_self, only_used_in_recursion, doc backticks) pre-date this stack on master and are left for a separate cleanup.

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