feat: add Mutation DSL for atomic bulk updates

[tecnosam]

Update

Introduces a Pythonic Mutation DSL that allows composing MongoDB atomic update operations ($set, $inc, $addToSet, $push, etc.) directly on field expressions.

Key features:

• Bulk updates via .find(...).mutate() / .amutate() without loading documents into memory
• Supports numeric ops (+=, *=, setmax/setmin), array ops (push, addToSet, pull, pop), nested fields, and unset
• Automatically collects mutations in a per-query context

Includes:

• MutationExpression & context management
• FieldExpression extensions for mutation syntax
• Safe usage patterns + warning about shared context
• Updated docs (updates-from-mutations.md/txt)

This is the main feature enabling efficient bulk maintenance, counters, rewards, and migrations.

No breaking changes — existing document → .save() workflow remains fully supported.