docs: implementation plan for input-validation pass

Six tasks (five commits + final verification) covering the five
spec items: charset parser inner-whitespace fix; Request.__post_init__
with URL, header/cookie, and mapping-field validation; Timeout/Limits
negative guards; ClientConfig.base_url validation + normalization plus
the redundant rstrip removal in AsyncClient._resolve_url; deferred-work
cleanup.

Each validation task is a TDD cycle (failing tests first, then
__post_init__ implementation). Task 2 has ~15 failing tests at once
because Request.__post_init__ enforces three related rules; introducing
them incrementally would mean three partial __post_init__ states.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: lesnik512