fix(launch): auto-detect minimum root volume size from AMI snapshot (#25)

[scttfrdmn]

Closes #25.

Problem

Launching from a custom AMI whose root snapshot is larger than spawn's 20 GiB default fails:

InvalidBlockDeviceMapping: Volume of size 20GB is smaller than snapshot 'snap-…', expect size >= 80GB

Users had to pass --volume-size 80 with a number they had to discover out of band. (Also the root cause behind nf-spawn#21.)

Fix

Launch now queries the AMI's BlockDeviceMappings (new rootVolumeSizeFromAMI helper) and floors the root volume at the snapshot minimum — final size is max(chosen, amiMin). This rescues:

• the default-20 case (custom AMI, no --volume-size), and
• an explicit --volume-size that's smaller than the snapshot.

Best-effort: any DescribeImages error (AMI not found, no permission) returns 0 and leaves the chosen size unchanged — a launch never fails because the lookup didn't work. Root device is matched by RootDeviceName, falling back to the largest EBS mapping as a safe floor.

Test

buildBlockDevices gains an amiMinGiB floor param; unit tests cover floor-raises-default, floor-beats-too-small-explicit, and larger-explicit-is-kept. Verified Tier 0 launch path unaffected (substrate's synthetic AMIs have no registered BDM, so the lookup returns 0 — no regression).

Follow-ups

• nf-spawn#21 can be closed once this ships — the ext.volumeSize workaround becomes unnecessary (spawn does the right thing automatically).
• A full Tier 0 assertion of the floor would need substrate to register an AMI with BlockDeviceMappings and return them from DescribeImages; will file that as a substrate enhancement. The unit tests cover the floor logic directly in the meantime.

[codecov]

Codecov Report

❌ Patch coverage is 51.28205% with 19 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
pkg/aws/client.go	51.28%	19 Missing ⚠️

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