refactor(core)!: Destination stores one partition at a time; PartitionedDestination folds into it

[aaaaahaaaaa]

Summary

Items 1 to 3 of the destination surface review.

• PartitionedDestination is gone. Every shipped destination extended it (CSV, File, Memory,
  GCS, and DatabaseDestination under BigQuery), while the Destination docstring and the first
  guide example taught the plain read/write shape that overwrites every partition. The scope
  template is now Destination itself: a destination is partition-correct by construction.
• The hooks are public. write_partition(context, partition, data) and read_partition(context, partition) are what a destination implements; a missing one raises naming itself. Overriding
  write/read wholesale stays possible for a backend that scopes differently, and is documented
  as that exception rather than as an alternative for everyone.
• The three-way dispatch exists once. IOContext.scopes lists the scopes a context covers
  ([None], [partition], or the window's partitions) and IOContext.slices(data) pairs each with
  its slice of the data. Destination.write and .read are a loop over them; the three copies of the
  None / Partition / PartitionWindow branching and their asserts are gone, including from
  DatabaseDestination.write, which keeps its window batching (clear every scope, insert once) and
  gains a write_partition for single scopes.

Migration

Before	After
class X(il.PartitionedDestination)	class X(il.Destination)
def _write_partition(...) / def _read_partition(...)	def write_partition(...) / def read_partition(...)
class X(il.Destination) overriding read/write for a partitioned asset	implement the two partition hooks instead

The guide, README, the destination skill and the upgrade skill's migration table are updated.

Verification

• tests/destination/test_partitioned.py folded into test_base.py on the public hooks; new
  test_context.py covers scopes, slices and the window-on-unpartitioned-asset error.
• Whole repo: 2820 passed, ruff and ty clean.

By Digitl

[codecov]

Codecov Report

❌ Patch coverage is 98.71795% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...rloper-core/src/interloper/destination/database.py	97.29%	1 Missing ⚠️

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