Adopt declscope, and align the file boundaries with the units

[mpyw]

declscope holds each declaration to the file that declares it, which is the private Go has no word for.

rules:
  naming:
    qualify: ondemand
    exported: true

No baseline

The first pass recorded 170 violations, 43 boundary and 127 naming. All of them are gone, and no baseline file is in this PR.

Where the boundary entries were

Package	What was crossing	What it turned out to be
internal/server	baseHandler reached from three files	One type split by concern. The three share a handler namespace now
internal/js	Transformer's compile and apply halves	One type again, joined in the core
internal/mock	Nine source-format entry points	A dispatcher with nine implementations, each entry declared with //declscope:package
e2e	Five helpers used by every test	Shared on purpose, declared at file level

closeQuietly was used from two namespaces and is now internal.CloseQuietly, since it belongs to neither.

Files renamed to match what they hold

statements.go became query.go, exec.go and bind.go. transform.go became compile.go, and Transforms became CompiledTransforms, which the doc comment already said. errors.go became error.go, because most of its declarations carry the singular.

Splitting statements.go cleared its entries without renaming a single declaration. The file name was the thing that was wrong.

Core, and what is left checked

One file per package, except internal/config where config.go and types.go are both the API and error.go remains a unit of its own with 16 declarations. No package is entirely core.

internal/cli/commands is the one place where core was the only option: Version is named by goreleaser's ldflags and cannot be renamed, and commands.App is the public entry.

Two vocabulary entries

handler: [handle] and responder: [response]. Both are word-form pairs the matcher cannot derive, which is declscope#64.

[codecov]

Codecov Report

❌ Patch coverage is 90.00000% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.90%. Comparing base (4b5ebf5) to head (e3402f8).

Files with missing lines	Patch %	Lines
internal/db/exec.go	70.00%	3 Missing ⚠️
internal/config/error.go	93.75%	1 Missing ⚠️
internal/db/bind.go	75.00%	1 Missing ⚠️
internal/helpers.go	50.00%	1 Missing ⚠️
internal/js/timeout.go	95.23%	1 Missing ⚠️
internal/server/handler.go	66.66%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #24   +/-   ##
=======================================
  Coverage   82.90%   82.90%           
=======================================
  Files          35       38    +3     
  Lines        1685     1685           
=======================================
  Hits         1397     1397           
  Misses        288      288           

Files with missing lines	Coverage Δ
internal/cli/commands/app.go	0.00% <ø> (ø)
internal/config/config.go	86.66% <ø> (ø)
internal/config/types.go	90.19% <100.00%> (+0.19%)	⬆️
internal/db/db.go	75.00% <ø> (ø)
internal/db/query.go	75.00% <ø> (ø)
internal/executor/compile.go	100.00% <100.00%> (ø)
internal/executor/executor.go	81.35% <ø> (ø)
internal/js/apply.go	90.36% <ø> (ø)
internal/js/error.go	88.46% <100.00%> (ø)
internal/js/helpers.go	90.90% <ø> (ø)
... and 21 more

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