Add Link 2C digital framing via Host PTZ

[p0ns]

Stack / review order

Depends on #1. Please merge the basic Link 2C support first.

This branch is stacked on feat/link2c-support. Both branches live in a fork, so GitHub cannot use that fork-only branch as the base of an upstream PR. This is a draft against upstream main; its full diff temporarily includes #1. After #1 merges, the branch can be rebased onto upstream main to leave only the framing commit.

Framing-only review: p0ns/linkctl@feat/link2c-support...feat/link2c-digital-framing

Framing commit: 1923457.

Summary

Add an explicit frame command for the fixed-lens Link 2C, separate from degree-based gimbal controls:

linkctl frame
linkctl frame --zoom 2 --center
linkctl frame --x 0.6
linkctl frame --y 0.4
linkctl frame --center
linkctl frame --json

• X/Y are normalized crop-center coordinates in 0..1; zoom is 1..4. Omitted fields retain their exact current values.
• Use the existing kernel UVC ioctl path, with no proprietary runtime dependency or driver detach.
• Validate model, extension GUIDs, command length, SET capability, finite input ranges, and activity before writing.
• Emit actual readback and a warning if the requested state has not been confirmed.
• Recognize the official SDK's 0x07 Auto Framing mode; reject unsupported/special/idle coordinate layouts.
• Add command/parser/protocol tests and document protocol evidence, usage, and safety constraints.

Protocol evidence

Recovered by static inspection of the official Insta360 Linux SDK, revision 2dd2c78186b0335a530042ae49accbef249d6a95. Full source links, binary SHA-256, symbol addresses, and decoding evidence are in docs/link2c-framing.md.

• Write: image XU (unit 10 on this camera), selector 0x13, SET_CUR; eight bytes: LE uint16 zoom/X/Y followed by movement/zoom step bytes.
• Position readback: info XU (unit 9), selector 0x02; LE uint16 X/Y at offsets 38/40, not the setter's selector.
• The SDK's GetHostPTZ does not populate zoom, so zoom is read separately through V4L2.
• Unit 9's mode payload is never written.

Hardware validation

On a physical Link 2C (2e1a:4c03):

• Coordinate/zoom readback works.
• Host PTZ GET_LEN returns 8 and GET_INFO returns 0x03 (GET/SET).
• The camera owner confirmed that framing writes work in manual testing. This is user-reported hardware validation, not an automated visual-motion assertion.
• Edge clamping, screen-direction polarity, timing, and interactions with Auto Framing still need broader validation. The overall model validation flag remains conservative.

Tests

• cargo fmt --check
• cargo test — 72 passed
• cargo test --features hardware-tests -- --ignored readonly — 5 passed
• cargo clippy --all-targets --all-features -- -D warnings
• Release build succeeds.