refactor: extract _with_flow_seq_fallback helper from append/extend_list

[nathanjmcdougall]

Addresses doc/todo/flow-sequence-fallback-duplication.md.

append() and extend_list() had identical try/except blocks: attempt the operation, catch PatchError, check for FLOW_SEQUENCE or NOT_A_SEQUENCE, fall back to get→mutate→replace. Extracted into _with_flow_seq_fallback(keys, patches, fallback_fn).

insert() is unchanged — it uses BLOCK_SEQUENCE_EXPECTED (a single kind covering both the flow and non-sequence cases) and needs an isinstance check, so it doesn't fit the same pattern.

Net: −22 lines, append and extend_list each collapse to two lines.

[nathanjmcdougall]

Closing this — on reflection the helper is too shallow to justify the indirection.

The function interface is clunky: callers still compute
oute for patch construction, then the helper recomputes it independently in the fallback path. insert() doesn't fit the pattern at all (different error kind + isinstance check), so the abstraction doesn't generalize. For only 2 call sites with slightly different lambdas, the inline duplication is actually easier to follow — the reader sees the full error-handling story in one place without jumping to a callback-based helper.

The original 9-12 line try/except blocks in �ppend and �xtend_list are straightforward and self-contained. The cost of the duplication is low; the cost of the indirection (lambda stack frames, split control flow, awkward parameter set) outweighs it.