docs: fix S3 adapter key encoding description and routing key placeholder consistency

[Copilot]

Two inaccuracies in the S3 adapter design doc: the segment encoding section described 0x00 as the terminator byte (it's actually the escape byte), and the routing key example used inconsistent placeholder names.

Changes

• Segment encoding (line 154): Corrected to match encodeDynamoKeySegment exactly — escape byte 0x00, raw 0x00 encoded as 0x00 0xFF, segment terminated by 0x00 0x01. Previous text implied 0x00 was the terminator, which would produce an incompatible encoder.

• Routing key placeholder (line 226): Standardized <bucket-enc><gen><object-enc> → <bucket-esc><gen-u64><object-esc> to match the naming convention used throughout the rest of the document.

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[bootjp]

@copilot what is esc?

[Copilot]

@copilot what is esc?

esc is short for escaped — it means the raw string value has been encoded using the DynamoDB adapter's key-segment escaping scheme:

• Any 0x00 byte in the original string is replaced by the two-byte sequence 0x00 0xFF
• The encoded segment is then terminated by 0x00 0x01

So <bucket-esc> means "the bucket name after applying that escaping", as opposed to the raw bucket name string. This distinguishes the encoded form from the raw input and from the numeric segments like <gen-u64> which use a different (fixed-width big-endian) encoding.