refactor: rename hdcp package to pipeline

[jravani]

Summary

internal/hdcp was a leftover from the project's original name (hdcp-go, before it became ignis) and read as an unrelated acronym next to the rest of the codebase -- exactly the kind of naming ambiguity we're trying to eliminate.

• Renamed internal/hdcp -> internal/pipeline (package hdcp -> pipeline). Matches the word already used throughout the docs/comments to describe this exact mechanism ("the 17-level pipeline") -- kept Pipeline/NewPipeline as-is rather than introducing a new name like Calculator, since the type's actual behavior (ordered stages, each depending on the last, with every intermediate level's result kept accessible via Lvl1...Lvl17) is precisely what "pipeline" describes; a name like "Calculator" would undersell that and would also collide with the neighboring internal/calc package and Calculate* methods.
• Renamed internal/service/hdcp_service.go -> ignis_service.go (+ test) to match the exported type it already contains, IgnisService.
• Fixed the one other importer, cmd/validate/main.go, which go build caught immediately.
• Updated data/README.md's one prose mention.

No exported symbol was named Hdcp/HDCP anywhere, and internal/ isn't importable outside this module, so this is a self-contained rename with no external impact.

Test plan

• go build ./..., go vet ./..., go test ./... all pass locally
• CI passes on this branch

[codecov-commenter]

Codecov Report

❌ Patch coverage is 82.35294% with 3 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
cmd/validate/main.go	0.00%	3 Missing ⚠️

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