Add mutations guide (#614)

* Add WIP version of mutations guide

* add before/after example

* docs: clean up mutation handler examples

- Pull mutation off mutations array for cleaner code
- Add comment explaining collection handlers only receive one mutation at a time
- Apply pattern consistently across all handler examples

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

* docs: remove pioneer analogy

* docs: fix mutation handler patterns

- Remove incorrect comment about single mutations
- Update handlers to use Promise.all for multiple mutations
- Fix collection.refetch() -> collection.utils.refetch()
- Update Electric collection pattern to return txids
- Add note about automatic refetch in collection handlers

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

* docs: fix temporary ID generator bug

Parenthesize the offset to ensure negative IDs: -(Math.floor(...) + 1)
Previously could produce positive 1 when Math.random() returns 0

* docs: clarify Immer-like pattern usage

Use 'Immer-like pattern' instead of 'uses Immer under the hood'

* docs: fix operation handler signatures

- Correct handler params to { transaction, collection }
- Remove incorrect metadata parameter from signature
- Add metadata to mutation object properties list
- Fix return type to Promise<any> | any

* docs: clarify handler sync requirements

Replace QueryCollection-specific note with general requirement that handlers
must wait for server changes to sync back before resolving

* docs: remove unnecessary refetch call in generic mutation function

Collection handlers automatically handle refetch, only custom actions need it

* docs: clarify commit timing with example

* docs: fix TransactionConfig options

- Make mutationFn required (not optional)
- Add missing id and metadata options
- Mark autoCommit as optional

* docs: clarify when to manually rollback transactions

Note that rollback is automatic on error, manual rollback only needed
for user actions like canceling a form

* docs: streamline mutation merging section

- Replace opening paragraph with bullet points
- Remove worked examples, keep truth table only
- Remove redundant benefits list

* Add an example.

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Co-authored-by: Claude <[email protected]>

Author: KyleAMathews