Default ignores cannot be removed, so engines under vendor/engines/ are silently unanalysable

[n-rodriguez]

Summary

DEFAULT_IGNORE_PATTERNS includes vendor/**/*, and Definition#add_ignore_patterns only
ever appends (@ignore_patterns |= …). There is no DSL affordance to drop a default ignore,
so a Rails app whose engines live in vendor/engines/ cannot analyse them at all.

This interacts badly with two other facts:

• DEFAULT_SOURCE_PATTERNS already lists engines/*/app/**/*.rb, so engines are clearly
  meant to be in scope — just not at the path Rails has historically used for them.
• Analyzer#ruby_files applies ignored as a final reject, after
  Definition#analysis_patterns has unioned in every component's file_patterns. So the DSL
  documentation ("Component patterns are always analyzed, so most projects never need this")
  does not hold against a default ignore: the component comes out empty and every rule on it
  is vacuously satisfied — the same shape as Analyse .rake files (source globs matching *.rake are silently dropped) #24.

Declaring source does not help either, and makes things worse:

def analysis_patterns
  patterns = source_patterns.empty? ? DEFAULT_SOURCE_PATTERNS.dup : source_patterns.dup
  patterns | component_specs.values.flat_map(&:file_patterns)
end

source replaces the defaults rather than adding to them, and the ignore still wins.

Measured

A Rails app with three tenant engines under vendor/engines/ (184 .rb files), on 1.1.0:

Spec	files reported	Engines analysed
components declared on vendor/engines/*/**/*.rb	3532 (= app + lib + spec exactly)	no
same, plus source "vendor/engines/**/*.rb"	2211 (defaults lost)	no
separate spec with root 'vendor/engines'	184	yes

archspec explain vendor/engines/engine_a/app/helpers/some_helper.rb returns
no file or constant found.

Two boundary rules — "no engine may reference another engine's namespace" and "the host app
may not call an engine operation" — reported 0 violations while checking nothing. They were
only caught because a deliberate violation was injected and stayed green.

Use case

each_directory 'vendor/engines/*' do |name, path|
  component name.to_sym, in: "#{path}/**/*.rb"
end

engine_a.cannot_use :engine_b, :engine_c

The root 'vendor/engines' workaround does analyse the engines, but that graph excludes the
host application, so any rule spanning host → engine is unexpressible in it.

Suggestion

Any of these would be enough:

1. let ignore remove a default, e.g. ignore '!vendor/**/*' or unignore 'vendor/engines/**/*';
2. exempt explicit component file_patterns from the default ignores (keeping user-declared
   ignores authoritative), which would match what the DSL docs already promise;
3. add vendor/engines/*/app/**/*.rb to DEFAULT_SOURCE_PATTERNS and narrow the default
   ignore to vendor/bundle/**/* and friends.

Either way, a component whose glob matches files on disk but resolves to zero analysed files
is worth a warning — that silence is what makes this class of bug survive.