docs: reflow architecture.md to reduce merge conflicts

[jussielo-amd]

Summary

• Reflow every paragraph in docs/architecture.md to one sentence per line (semantic line breaks) — markdown collapses single newlines to a space, so rendering is unchanged.
• Convert each per-crate section's module narrative into bullet lists (one bullet per extracted file), instead of one long run-on paragraph per crate.
• Why: every ### crate section was a single very long markdown line. Git merges per line, so two PRs editing the same crate's section — common during EAI-7768's modularization effort, which lands "one cluster at a time" as sequential PRs against the same crate (e.g. apps/rocmd's persistence.rs PR ROCMAI-83: extract persistence.rs from apps/rocmd/src/lib.rs #477 then common.rs PR ROCMAI-83: extract common.rs, webhook.rs, and cli.rs from apps/rocmd/src/lib.rs #479) — collided on that one line even though their actual edits were unrelated sentences. Smaller diff units (sentence/bullet instead of paragraph) make those edits mergeable without manual conflict resolution.
• Fixes one pre-existing stale claim found during review: the common.rs entry said it holds "small arg/healthcheck/endpoint-key utilities," but parse_gpu_indices_arg/optional_arg were moved back to lib.rs in that same extraction PR's (ROCMAI-83: extract common.rs, webhook.rs, and cli.rs from apps/rocmd/src/lib.rs #479) "Review fixup" for not meeting the module's >=2-caller admission rule — the doc wording wasn't updated to match. Dropped "arg/"; the healthcheck/endpoint-key claims are accurate. This predates this PR (confirmed identical wording on origin/main) — fixed here as a one-line drive-by since the line was already being touched.
• Everything else: no factual content changed — this is a formatting pass. Word-diffed the change to confirm no other claim was altered, only structure (parens → em-dash, bullets instead of inline lists).
• Risk: low. Docs-only change, no code touched, verified against the CI gate that actually enforces this doc's correctness (below).

Test plan

• cargo xtask check-architecture-doc passes (this is what CI's architecture-doc job runs).
• cargo test -p xtask architecture_doc — all 48 unit tests pass, including run_passes_against_the_real_doc.
• Manually word-diffed before/after to confirm only formatting changed (plus the one documented content fix above).

Checklist

• N/A — no bug fix (no xfail rows to update).
• N/A — no new subcommand/subsystem added.
• N/A — no user-facing message changed.

[Copilot]

Copilot review overview

🟢 Approval recommended

The documentation-only restructuring is accurate and introduces no unresolved issues.

Review effort: Balanced
Findings: None

What changed in this PR

Reformats the architecture guide to reduce merge conflicts while preserving its meaning and correcting one stale common.rs description.

Changes:

• Applies sentence-level line breaks.
• Converts module narratives into structured lists.
• Removes the outdated claim that common.rs contains argument utilities.

File	Description
docs/​architecture.md	Reflows and restructures the module map for conflict-resistant editing.

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