docs: restore specification overview section order

[amithanda]

The specification overview opened with ~780 lines of deep normative detail
before the reader reached the architectural introduction. Anyone landing on
the spec met "Quantities and units" as their first content.

This happened because three PRs each inserted a new cross-cutting primitive at
the top of the file, immediately after "Schema notes":

PR	Section	Inserted at
#582	Actions	line 34
#653	Quantities and units	line 34
#655	Request Constraints	line 175

Each insertion was individually reasonable, since these are foundational
vocabulary that applies everywhere. The changes were cumulative: each later
insertion pushed the earlier one down, stacking all three ahead of "Discovery,
Governance, and Negotiation", which is the actual entry point for a first-time
reader.

Worth noting that #572, the earliest of these four primitives, placed Policies
after "Standard Capabilities". This PR follows that precedent and moves the
other three to join it, so all four core primitives sit together:

This is a pure reorder. No prose edits, no heading level changes. Verified
by comparing the sorted line multiset against the parent commit: identical,
same 3747 total lines. All 142 heading anchors are preserved, so the 20+ pages
that deep-link into #quantities-and-units, #request-constraints and
#actions are unaffected.

Reviewing with git diff --color-moved or ignoring whitespace makes the move
much easier to read than the raw 2542/2542 diff.

Category (Required)

• Documentation: Updates to README, or documentations regarding schema or capabilities. (Requires Maintainer approval)

Related Issues

Checklist

• I have followed the Contributing Guide (including Conventional Commits title requirements and ! for breaking changes).
• I have updated the documentation (if applicable).
• My changes pass all local linting and formatting checks.
• I have added tests that prove my fix is effective or that my feature works. — n/a, no code change
• New and existing unit tests pass locally with my changes.
• (For Core/Capability) I have included/updated the relevant JSON schemas. — n/a, no schema change
• I have regenerated Python Pydantic models by running generate_models.sh under python_sdk. — n/a, no schema change

Screenshots / Logs (if applicable)

Before (current main), /specification/overview/:

/ After (this PR):

Note: this PR fixes /draft/. The /latest/ alias resolves to the
2026-08-25 release, so restoring the live /latest/ URL additionally
requires backporting this commit to release/2026-08-25 — will send via a future PR.