feat(processing): pick the Reference source position on the spectrum

[Limdongcheng]

Summary

The Reference step's source position could only be typed as a number, while the value the user wants is "the peak currently sitting there" — visible on the plot but read off manually.

The step editor gains a "Pick position on spectrum" row that arms a one-shot on-plot pick. While armed, hovering the target plot previews the position with a dashed guide line, an apex dot and a ppm readout; the pick reuses the zoom-scaled apex snap manual peak picking uses (Shift = nearest sample), so weak lines are reachable by zooming in first. A click writes the step's at_ppm through the property catalog — one undo step, the same recompute as typing — and prompts for the target position. Esc, collapsing the editor, or switching datasets disarms; the arm state is revalidated per frame like the on-plot phase mode, so a collapsed card never leaves a live click trap.

The picked coordinate lives on the finished axis while at_ppm lives on the axis entering the step, so the commit subtracts the calibration applied at or after the step — exposed as AxisPipeline::chemical_shift_offset_from_step_ppm beside the existing whole-pipeline reduction, keeping reference-step semantics in one place. After the edit the picked feature reads exactly target_ppm even when re-picking a step that already carries an offset.

canvas/mod.rs's inline test modules move to a sibling mod_tests.rs, and ui_state.rs's dialog-state cluster to ui_state_dialogs.rs, to stay under the 800-line source limit.

Validation

• cargo pr-check passes on current main (fmt, source sizes, dependency policy, default-frontends build, clippy -D warnings, reference-backend test suite).
• Tests: processing-level offset-from-step reduction; pick arm/resolve lifecycle (valid only while the arming editor stays expanded on the active dataset); end-to-end commit conversion including undo.
• The Processing manual page (English and zh-CN) gains a Reference section documenting the workflow; npm run build in docs/ passes.

UI display logic

• New or changed actions go through the command catalog (CommandId + describe) and are searchable in the command palette. (The pick button is a panel-local widget inside the step editor — explicitly exempt in the catalog rules; no commands added.)
• Hiding happens only at Ribbon-group level (dataset kind or width budget); transient states disable with a disabled_reason that says how to unblock. (No show/hide logic touched.)
• No layout changes from background events; panels are never auto-closed. (The pick mode is revalidated per frame against the editor's expanded state; it never opens or closes panels.)
• New panels or empty regions show an empty state with a next step. (No new panels.)
• Docs updated in docs/ (English and zh-CN) for user-visible behavior. (Processing guide gains a Reference section in both languages.)