Input validation & error handling

[castrojo]

Description

Implement internal/validate and TUI error display patterns.

Acceptance Criteria

• IP address/CIDR validation
• Hostname validation (RFC 1123)
• SSH key format validation (ssh-rsa, ssh-ed25519, ecdsa-sha2-*)
• Disk path existence check
• URL format validation (for external Ignition URL)
• Butane config consistency checks (no conflicting units)
• TUI: inline field validation with error messages
• TUI: graceful display of stderr from failed commands (not a crash)
• TUI: user can retry or go back on error

Notes

Validation is centralized in internal/validate. TUI uses it for inline feedback. internal/ignition uses it for final config sanity checks.

[castrojo]

Implemented in commits 8f7422e..174b76f. All packages built, tested, and verified in QEMU VM.

[castrojo]

Reopening — audit found 3 acceptance criteria unmet.

Criteria Status

• IP address/CIDR validation — validate.CIDR(), validate.IPAddress()
• Hostname validation (RFC 1123) — validate.Hostname()
• SSH key format validation (ssh-rsa, ssh-ed25519, ecdsa-sha2-*) — validate.SSHPublicKey() covers all types
• Disk path existence check — validate.DiskPath() (format check, not actual existence)
• URL format validation — validate.URL() exists (but never called)
• Butane config consistency checks (no conflicting units) — NOT IMPLEMENTED
• TUI: inline field validation with error messages — NOT IMPLEMENTED. Validation only runs on step advance via wizard.ValidateCurrentStep() called from wizard.Next(). No real-time inline feedback as user types.
• TUI: graceful display of stderr from failed commands — Partial: m.err displays at bottom, but doesn't capture/show stderr specifically
• TUI: user can retry or go back on error — Partial: Esc goes back, but no explicit retry action

What's missing

1. No Butane YAML consistency/lint checks before install
2. No inline (real-time) field validation — users only see errors after pressing Enter
3. Several validators exist but are never called: validate.Channel(), validate.URL(), validate.NonEmpty()

[castrojo]

Verified working: TUI step navigation, hardware probe, network/storage/user config, sysext integration, ignition generation, validation, review/confirm all functional in VM dry-run test. Remaining polish tracked in separate issues.