feat(FR-001): implement Core Engine

[andhijeannot]

What

Implements Feature Requirement FR-001: Core Engine for the `htload` load-testing tool.

Includes

• Engine types (`pkg/engine/types.go`): Scenario, Phase, Step, Result, StepContext, BodySource, RateProfile, Ramp, Assertion, Capture
• Driver interface (`pkg/engine/driver.go`): Name, Execute, Close — protocol-agnostic contract
• Reporter interface (`pkg/engine/reporter.go`): Full lifecycle event callbacks
• HTTP driver (`pkg/http/driver.go`): GET/POST/PUT/DELETE/PATCH/HEAD/OPTIONS with status-code assertions, template interpolation for path/body/headers, file body support
• Runner (`pkg/engine/runner.go`): Multi-phase VU orchestration, panic recovery, fail-fast loop termination, setup/teardown execution
• ConsoleReporter (`pkg/reporter/console.go`): `log/slog`-based JSON output with ✓/✗ markers, phase/scenario summaries
• Template funcmap (`internal/template/interpolate.go`): UUID, RandomString, Timestamp, VU.ID, VU.Iteration, Env, Vars
• Metrics aggregation (`internal/metrics/metrics.go`): Percentile, Mean, Min, Max, Median

Quality Gates

Gate	Result
Tests pass	✓ `go test ./...`
Race clean	✓ `go test -race ./...`
Lint clean	✓ `golangci-lint run ./...` — 0 issues
gofumpt	✓ formatted
Coverage	engine 83.7%, http 95.7%, reporter 100%, template 84.6%, metrics 91.5%
Import boundary	✓ `pkg/engine` has zero `net/http` imports

Bug fixed

• `resolveBody` now interpolates `Inline` and `File` sources (previously only `Template`)