folding with overlays

[mahmoodsh36]

this is an attempt to get folding support using overlays with the :invisible attribute and other special attributes for overlays. in emacs there are 2 approaches for folding that im aware of, first is using text attributes and the second uses overlays. this approach is inspired by the second.

currently :invisible, :before-string, :after-string have been implemented with the most minimal changes possible. efficiency wasnt taken into account.

reference for emacs overlay attributes

[code-contractor-app]

✅ Code Contractor Validation: PASSED

=== Contract: contract ===

✓ Code Contractor Validation Result
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📋 Contract Source: Repository

📊 Statistics:
  Files Changed:    2
  Lines Added:      257
  Lines Deleted:    85
  Total Changed:    342
  Delete Ratio:     0.25 (25%)

Status: PASSED ✅

🤖 AI Providers:
  - codex — model: gpt-5.4

━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
🎉 No violations detected. Great job!

ℹ️ PASS Notes (1): low-confidence checks — please sanity-check the reasoning.

[NOTE] defpackage_rule (confidence: 63)
  Reason:
    The diff adds no package form, so there is no added code violating the
    defpackage/package-name requirement.

📋 Contract Configuration: contract (Source: Repository)

version: 2

trigger:
  paths:
    - "extensions/**"
    - "frontends/**/*.lisp"
    - "src/**"
    - "tests/**"
    - "contrib/**"
    - "**/*.asd"
  head_branches:
    exclude:
      - 'revert-*'

validation:
  limits:
    max_total_changed_lines: 400
    max_delete_ratio: 0.5
    max_files_changed: 10
    severity: warning

  ai:
    system_prompt: |
      You are a senior Common Lisp engineer reviewing code for Lem editor.
      Lem is a text editor with multiple frontends (ncurses, SDL2, webview).
      Focus on maintainability, consistency with existing code, and Lem-specific conventions.
    rules:
      # === File Structure ===
      - name: defpackage_rule
        prompt: |
          First form must be `defpackage` or `uiop:define-package`.
          Package name should match filename (e.g., `foo.lisp` → `:lem-ext/foo` or `:lem-foo`).
          Extensions must use `lem-` prefix (e.g., `:lem-python-mode`).

      - name: file_structure_rule
        prompt: |
          File organization (top to bottom):
          1. defpackage
          2. defvar/defparameter declarations
          3. Key bindings (define-key, define-keys)
          4. Class/struct definitions
          5. Functions and commands

      # === Style ===
      - name: loop_keywords_rule
        prompt: |
          Loop keywords must use colons: `(loop :for x :in list :do ...)`
          NOT: `(loop for x in list do ...)`

      - name: naming_conventions_rule
        prompt: |
          Naming conventions:
          - Functions/variables: kebab-case (e.g., `find-buffer`)
          - Special variables: *earmuffs* (e.g., `*global-keymap*`)
          - Constants: +plus-signs+ (e.g., `+default-tab-size+`)
          - Predicates: -p suffix for functions (e.g., `buffer-modified-p`)
          - Do NOT use -p suffix for user-configurable variables

      # === Documentation ===
      - name: docstring_rule
        prompt: |
          Required docstrings for:
          - Exported functions, methods, classes
          - `define-command` (explain what the command does)
          - Generic functions (`:documentation` option)
          Important functions should explain "why", not just "what".
        severity: warning

      # === Lem-Specific ===
      - name: internal_symbol_rule
        prompt: |
          Use exported symbols from `lem` or `lem-core` package.
          Avoid `lem::internal-symbol` access.
          If internal access is necessary, document why.

      - name: error_handling_rule
        prompt: |
          - `error`: Internal/programming errors
          - `editor-error`: User-facing errors (displayed in echo area)
          Always use `editor-error` for messages shown to users.

      - name: frontend_interface_rule
        prompt: |
          Frontend-specific code must use `lem-if:*` protocol.
          Do not call frontend implementation directly from core.
        severity: warning

      # === Functional Style ===
      - name: functional_style_rule
        prompt: |
          Prefer explicit function arguments over dynamic variables.
          Avoid using `defvar` for state passed between functions.
          Exception: Well-documented cases like `*current-buffer*`.

      - name: dynamic_symbol_call_rule
        prompt: |
          Avoid `uiop:symbol-call`. Rethink architecture instead.
          If unavoidable, document the reason.

      # === Libraries ===
      - name: alexandria_usage_rule
        prompt: |
          Alexandria utilities allowed: `if-let`, `when-let`, `with-gensyms`, etc.
          Avoid: `alexandria:curry` (use explicit lambdas)
          Avoid: `alexandria-2:*` functions not yet used in codebase

      # === Macros ===
      - name: macro_style_rule
        prompt: |
          Keep macros small. For complex logic, use `call-with-*` pattern:
          ```lisp
          (defmacro with-foo (() &body body)
            `(call-with-foo (lambda () ,@body)))
          ```
          Prefer `list` over backquote outside macros.

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[mahmoodsh36]

currently there is a confusion between the concept of a 'logical-line' and a 'visual-line' in the code. especially the code in logical-line.lisp could do with some renaming.

also, performance is bad. this PR doesnt address performance issues, atleast not yet.