feat(config): add hot-reload extension points (#1619)

[lzjqsdd]

Summary

Sixth PR of the stacked series for #1619 (6/7). Adds hot-reload extension points only — no watcher or admin RPC is wired yet.

Issue Describe / Design

Related to #1619 (partial). Depends on the preceding stacked PRs.

• Reloadable marker: sections safe to swap at runtime (first implementation: LogConf).
• RuntimeOverlay: ephemeral admin-pushed dotted-path overrides, deliberately lost on restart.
• ConfigHandle: Arc-snapshot holder; readers clone the Arc, so a swap never tears an in-flight consumer. No new dependencies.
• Module docs describe the two future mechanisms: file-watch reload and admin-API pushdown — both are direct follow-ups since the pipeline is a pure function from layers to a validated ClusterConf.

Changes

Module / File	Change	Impact on existing behavior
crates/common/curvine-config/src/reload.rs (new)	Extension-point types + docs	None (no production callers yet)
crates/common/curvine-config/src/lib.rs	Export reload module	None

Test verified

Test case	Result	Notes
cargo test -p curvine-config	PASS	Overlay set/replace/clear; handle swap leaves old snapshots untouched

Dependencies

• Related PRs: base refactor(config): converge config discovery on ConfigLoader (#1619) #1625
• Related issues: [FEATURE] Unify component configuration loading, validation, and hot-reload extension points #1619

[szbr00]

Re-reviewed head c96ee5b against base unified-conf-p5. Dirty crate: curvine-config (reload.rs + lib.rs export).

Previous findings on apply_to error swallowing, LogConf Reloadable, and overlay rank are addressed: apply_to returns CommonResult and the collision test fails closed; LogConf is a documented future candidate only; apply_to and the module Effective layer order match pipeline.rs (file → env → overlay → deserialize).

Inspected overlay set/replace/clear and snapshot isolation tests, the set_dotted collision path, ConfigHandle Arc swap (lock not held after get), no production callers (intentional per #1619), no proto/SDK/lockfile changes, and no data-path impact.

One leftover: RuntimeOverlay / Admin-API docs still say "above the file layer" (see inline). Not blocking.