feat(rustfoil-cli): add --flap option to polar and faithful-polar

[aeronauty-flexcompute]

Summary

• Adds an XFOIL-style --flap "hinge_x:hinge_y:deflection_deg" option to both the inviscid polar and viscous faithful-polar commands.
• The deflection is applied via the existing rustfoil_core::flap::xfoil_flap before the body is built. faithful-polar writes the flapped section to a temp .dat and feeds it to XFOIL.
• Lets you generate flapped-airfoil priors straight from the CLI — no separate pre-deflected coordinate file needed.

Motivation

Came out of the blown-lift / Tailor multifidelity work (FXC-9009): we needed a cheap flapped-airfoil lift prior for an LS(1)-0417 with a deployed flap. A plain-XFOIL flap on the main element turns out to be a strong prior — the viscous+flap panel curve carries ~98.5% of the RANS C_L variance, leaving Tailor to model only the small residual.

Test Plan

# inviscid, 12 deg flap at hinge (0.72c, 0.5 t)
rustfoil polar ls417.dat --alpha-start=0 --alpha-end=12 --alpha-step=4 --flap 0.72:0.5:12
# viscous (XFOIL) with the same flap
rustfoil faithful-polar ls417.dat --re 1e7 --mach 0.2 --alpha 0:12:4 --flap 0.72:0.5:12

Verified locally: flap increases C_L monotonically with deflection; a bad --flap string warns and is ignored (clean polar still runs).

🤖 Generated with Claude Code

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Note

Low Risk
CLI-only wiring around existing flap geometry; invalid specs are ignored with a warning and default behavior is unchanged.

Overview
Adds --flap "hinge_x:hinge_y:deflection_deg" to the inviscid polar and viscous faithful-polar subcommands so flapped polars can be run without a pre-deflected coordinate file.

When --flap is set, coordinates are transformed with rustfoil_core::flap::xfoil_flap before Body construction (inviscid) or before XFOIL-style repaneling via a new build_body_for_faithful_points helper (faithful path). A shared parse_flap_spec parses the colon-separated triple; malformed values log a warning and the command falls back to the unflapped geometry.

Status messages about the applied hinge/deflection go to stderr so tabular polar output stays clean.

^{Reviewed by Cursor Bugbot for commit 62b263e. Bugbot is set up for automated code reviews on this repo. Configure here.}

[cursor]

Cursor Bugbot has reviewed your changes and found 3 potential issues.

Bugbot Autofix prepared fixes for all 3 issues found in the latest run.

• ✅ Fixed: Invalid flap silently ignored
  • faithful-polar now uses shared flap validation and emits the same bad --flap warning before ignoring malformed input.
• ✅ Fixed: Flap fields shift after empty segments
  • Both polar paths now require exactly three colon-separated flap fields and reject any unparseable or empty segment.
• ✅ Fixed: Shared temp file race
  • faithful-polar now builds the flapped body directly from in-memory points instead of writing a shared temporary geometry file.

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^{Reviewed by Cursor Bugbot for commit 9694a76. Configure here.}